Instrumentation

1.
11/10/2008
Instrumentation - II | Er. Sharib Ali
ER. SHARIB
ALI INSTRUMENTATION SYSTEM-II

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From the Author
“A complete manual on INSTRUMENTATION-II” has been written with the
motive of making the students of Engineering build a concept on different areas of the
electronics based instrumentation systems vastly used in medical instruments and industrial
instruments. The concept of manual is also to emphasis on the various sensors and
transducers, amplifiers, interfacing devices like ADC, DAC, multiplexers, decoders etc
which are essential in the design of any microprocessor based instrumentation. It also covers
the several algorithms, flow charts and software concepts used in the programming of the
microprocessor based instrumentation system. The manual is divided into three parts-
I. Biomedical Instrumentation
II. Microprocessor Based Instrumentation and its components and
III. Various case studies related to the Industrial troubleshooting, case study
preparation and techniques to implement advanced system for better results
both economically and technically.
The manual is concise in itself covering the syllabus of Instrumentation II (BEG 434EC)
of VIIIth semester of Purvanchal University.
I am highly obliged to Er. Ashok Yadav (Principal of Eastern College of Engineering), Er.
Ghanshyam Pathak (Head of Electronics Department), Er. B.M. Singh (Program co-
coordinator of EASCOLL) and all my students of VIIIth semester especially Mr. Arjun
Bhandary, Mr. Robin Pokhrel, Mr. Sunil Singh, Mr. Gyannath Sapkota, Mr. Bhupal
Khatiwada, Miss. Kamu Aryal, Mr. Jayendra Mehta and all others who encouraged me
to compile this manual.
I want to thank my family members who have supported me during my late works.
I look forward to your kind suggestion towards improvement.
Er. Sharib Ali

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What is a biomedical instrument?
To many it’s an EKG machine, to others it’s a chemical biosensor, and to some it’s a
medical imaging system. There are a lot of instruments that qualify as biomedical
instruments. Current estimates of the worldwide market in biomedical instruments is over
$200 billion.
Even though there is a wide variety of instruments, almost all of them can be modeled using
the simple diagram below.
Biomedical Instrumentation System
All biomedical instruments must interface with biological materials. That interface can by
direct contact or by indirect contact (e.g., induced fields).
A sensor must:
i. detect biochemical, bioelectrical, or biophysical parameters
ii. reproduce the physiologic time response of these parameters
iii. provide a safe interface with biological materials
An actuator must:
i. deliver external agents via direct or indirect contact
ii. control biochemical, bioelectrical, or biophysical parameters
iii.provide a safe interface with biologic materials
The electronics interface must:
i. match electrical characteristics of the sensor/actuator with computation unit
ii. preserve signal to noise ratio of sensor
iii. preserve efficiency of actuator
iv. preserve bandwidth (i.e., time response) of sensor/actuator
v. provide a safe interface with the sensor/actuator
vi. provide a safe interface with the computation unit
vii. provide secondary signal processing functions for the system

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The computation unit must:
i. provide primary user interface; ii. provide primary control for the overall system
ii. data storage for the system; provide primary signal processing functions for the
system;
iii. maintain safe operation of the overall system
Types of Biomedical Instruments
Types of Biomedical Instrumentation Systems
• Direct / Indirect
• Invasive / Noninvasive
• Contact / Remote
• Sense / Actuate
• Real-time / Static
Direct/Indirect - The sensing system measure a physiologic parameter directly, such as the
average volume blood flow in an artery, or measures a parameter related to the physiologic
parameter of interest (e.g., EKG recording at the body surface is related to propagation of
the action potential in the heart but is not a measurement of the propagation waveform).
Invasive/Noninvasive - Direct electrical recording of the action potential in nerve fibers
using an implantable electrode system is an example of an invasive sensor. An imaging
system measuring blood flow dynamics in an artery (e.g., ultrasound color flow imaging of
the carotid artery) is an example of a noninvasive sensor.
Contact/Remote - A strain gauge sensor attached to a muscle fiber can record deformations
and forces in the muscle. An MRI or ultrasound imaging system can measure internal
deformations and forces without contacting the tissue.
Sense/Actuate - A sensor detects biochemical, bioelectrical, or biophysical parameters. An
actuator delivers external agents via direct or indirect contact and/or controls biochemical,
bioelectrical, or biophysical parameters. An automated insulin delivery pump is an example
of a direct, contact actuator. Noninvasive surgery with high intensity, focused ultrasound
(HIFU) is an example of a remote, noninvasive actuator.
Real-time/Static - Static instruments measure temporal averages of physiologic parameters.
Real-time instruments have a time response faster than or equal to the physiologic time
constants of the sensed parameter. For example a real-time, ultrasound Doppler system can
measure changes in arterial blood velocity over a cardiac cycle.

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Testing Instrumentation
Chapter-1
1.0Factors to be considered in the design or specification of medical
instrumentation system-
The branch of science that includes the measurement of physiological variables and
parameters is known as biometrics. Biomedical instrumentation provides the tools by which
these measurements can be achieved.
In the design or specification of medical instrumentation systems, each of the following
factors should be considered.
i) Range: The range of an instrument is generally considered to include all the
levels of input amplitude and frequency over which the device is expected to
operate.
ii) Sensitivity: The sensitivity of an instrument determines how small variation of a
variable or parameter can be reliably measured. The sensitivity is concerned with
the minute changes that can be detected. Too high sensitivity often results in
nonlinearities or instability. Thus, the optimum sensitivity must be determined
for any given type of measurement.
iii) Linearity: The degree to which variations in the o/p of an instrument follow i/p
variations is referred to as the linearity of the device. In a linear system the
sensitivity would be the same for all absolute levels of i/p whether in the high,
middle or low portion of the range. Linearity should be obtained over the most
important segments even if it is impossible to achieve it over the entire range.
iv) Hysteresis: Hysteresis is a characteristic of some instruments whereby a given
value of the measured variable results in a different reading when reached in an
ascending direction from that obtained when it is reached in a descending
direction.
v) Frequency Response: The frequency response of an instrument is its variation
in sensitivity over the frequency range of the measurement. It is important to
display a wave shape that is a faithful reproduction of the original physiological
signal.
vi) Accuracy: Accuracy is a measure of systemic error. Accuracy is the difference
between true value and measured value divided by true value.
vii) Signal to noise ratio (SNR): It is important that the SNR be as high as possible.
It is also important to know and control the SNR in the actual environment in
which the measurements are to be made.
viii) Stability: In control engineering, stability is the ability of a system to resume a
steady state condition following a disturbance of the i/p rather than a driven
uncontrollable oscillation.
ix) Isolation: Often measurements must be made on patients or experimental
animals in such a way that the instrument does not produce a direct electrical
connection between the subject and ground.
x) Simplicity: All systems and instruments should be as simple as possible to
eliminate the change of component or human error.

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1.1 Microprocessors in Biomedical Instrumentation
The first biomedical instruments incorporating microprocessors began to appear on the
market around 1975 while the first devices were mainly laboratory type instruments,
microprocessors are now used in all areas of biomedical instrumentation.
Following are some examples of the ways in which microprocessors are employed in
contemporary medical instruments.
i) Calibration: Many instruments require zeroing and recalibration at certain time
intervals, sometime every few hours. A software or hardware timer in a
microprocessor system can initiate a calibration cycle. Microprocessor equipped
devices usually perform the calibration in digital form. During the calibration,
offset and gain correction factors are determined and stored in memory to be
applied to the measured data during the measurement.
ii) Table lookup: In analog systems, non-linear functions are usually implemented
by straight line approximations. In microprocessor equipped systems, table
lookup with interpolation can be used. This procedure is less limited and more
accurate and also permits the determination of parameters that are dependent on
more than one variable.
iii) Averaging: Microprocessor can easily average data over time or over successive
measurements and can thus decrease statistical variations.
iv) Formatting and printout: Because medical equipment using microprocessor
usually processes data in digital form, the microprocessor can be utilized to
format the data, convert the raw data into physical units and print out the results
in a form that does not require further transcribing or processing.
1.1.1 Introduction to infrared, Ultraviolet and X-ray
Infrared (IR) radiation is electromagnetic radiation whose wavelength is longer
than that of visible light, but shorter than that of terahertz radiation and microwaves. The
name means "below red" (from the Latin infra, "below"), red being the color of visible light
with the longest wavelength. Infrared radiation has wavelengths between about 750 nm and
1 mm, spanning three orders of magnitude. Humans at normal body temperature can radiate
at a wavelength of 10 microns.
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than
that of visible light, but longer than X-rays. It is named because the spectrum consists of
refrangible electromagnetic waves with frequencies higher than those that humans identify
as the color violet.
UV light is typically found as part of the radiation received by the Earth from the Sun. Most
humans are aware of the effects of UV through the painful condition of sunburn. The UV
spectrum has many other effects, including both beneficial and damaging changes to human
health.

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An X-ray (or Röntgen ray) is a form of electromagnetic radiation with a wavelength
in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30
petahertz to 30 exahertz (30×1015Hz to 30×1018Hz) and energies in the range 120 eV to
120 keV. They are longer than gamma rays but shorter than UV rays. X-rays are primarily
used for diagnostic radiography and crystallography. X-rays are a form of ionizing radiation
and as such can be dangerous. In many languages it is called Röntgen radiation after one of
the first investigators of the X-rays, Wilhelm Conrad Röntgen.
1.1.2 Application of IR, UV and X-rays
Applications of IR
1. Infrared Filters
Infrared (IR) filters can be made from many different materials. Infrared filters allow a
maximum of infrared output while maintaining extreme covertness. Currently in use around
the world, infrared filters are used in Military, Law Enforcement, Industrial and Commercial
applications. All generations of night vision devices are greatly enhanced with the use of IR
filters.
2. Night Vision Equipment
Infrared is used in night vision equipment when there is insufficient visible light to see.
Night vision devices operate through a process involving the conversion of ambient light
photons into electrons which are then amplified by a chemical and electrical process and
then converted back into visible light. Infrared light sources can be used to augment the
available ambient light for conversion by night vision devices, increasing in-the-dark
visibility without actually using a visible light source.
3. Thermography
Infrared radiation can be used to remotely determine the temperature of objects (if the
emissivity is known). This is termed thermography. Thermography (thermal imaging) is
mainly used in military and industrial applications but the technology is reaching the public
market in the form of infrared cameras on cars due to the massively reduced production
costs.
4. Communications
IR does not penetrate walls and so does not interfere with other devices in adjoining rooms.
Infrared is the most common way for remote controls to command appliances. Free space
optical communication using infrared lasers can be a relatively inexpensive way to install a
communications link in an urban area operating at up to 4 gigabit/s, compared to the cost of
burying fiber optic cable.
Infrared lasers are used to provide the light for optical fiber communications systems.
Infrared light with a wavelength around 1,330 nm (least dispersion) or 1,550 nm (best
transmission) are the best choices for standard silica fibers.

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5. Meteorology
Weather satellites equipped with scanning radiometers produce thermal or infrared images
which can then enable a trained analyst to determine cloud heights and types, to calculate
land and surface water temperatures, and to locate ocean surface features. The scanning is
typically in the range 10.3-12.5 µm (IR4 and IR5 channels).
Applications of UV
1. Security
To help thwart counterfeiters, sensitive documents (e.g. credit cards, driver's licenses,
passports) may also include a UV watermark that can only be seen when viewed under a
UV-emitting light. Passports issued by most countries usually contain UV sensitive inks and
security threads. Visa stamps and stickers on passports of visitors contain large and detailed
seals invisible to the naked eye under normal lights, but strongly visible under UV
illumination.
2. Fluorescent lamps
Fluorescent lamps produce UV radiation by ionising low-pressure mercury vapour. A
phosphorescent coating on the inside of the tubes absorbs the UV and converts it to visible
light.
3. Spectrophotometry
UV/VIS spectroscopy is widely used as a technique in chemistry, to analyze chemical
structure, most notably conjugated systems. UV radiation is often used in visible
spectrophotometry to determine the existence of fluorescence in a given sample.
4. Sterilization
A low pressure mercury vapor discharge tube floods the inside of a hood with shortwave
UV light when not in use, sterilizing microbiological contaminants from irradiated surfaces.
Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and
medical facilities.
5. Lasers
Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology
and keratectomy), free air secure communications and computing (optical storage). They
can be made by applying frequency conversion to lower-frequency lasers, or from
Ce:LiSAF crystals (cerium doped with lithium strontium aluminum fluoride).

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Applications of X-Ray
• Medical diagnosis: Since Röntgen's discovery that X-rays can identify bony
structures, X-rays have been developed for their use in medical imaging. Radiology
is a specialized field of medicine. Radiographers employ radiography and other
techniques for diagnostic imaging. This is probably the most common use of X-ray
technology.
• X-ray crystallography - in which the pattern produced by the diffraction of X-rays
through the closely spaced lattice of atoms in a crystal is recorded and then analyzed
to reveal the nature of that lattice. A related technique, fiber diffraction, was used by
Rosalind Franklin to discover the double helical structure of DNA.
• X-ray astronomy, which is an observational branch of astronomy, which deals with
the study of X-ray emission from celestial objects.
• X-ray microscopic analysis, which uses electromagnetic radiation in the soft X-ray
band to produce images of very small objects.
• X-ray fluorescence, a technique in which X-rays are generated within a specimen
and detected. The outgoing energy of the X-ray can be used to identify the
composition of the sample.
• Industrial radiography uses x-rays for inspection of industrial parts, particularly
welds.
• Paintings are often X-rayed to reveal the underdrawing and pentimenti or alterations
in the course of painting, or by later restorers. Many pigments such as lead white
show well in X-ray photographs.
• Airport security luggage scanners use x-rays for inspecting the interior of luggage
for security threats before loading on aircraft.
1.1 X-RAY
Basis of Diagnostic Radiology:
A radiological examination is one of the most important diagnostic aids in the
medical practice. It is based on the fact that various anatomical structures of the body have
different densities for the X-rays.
When X-rays from a point source penetrate a section of the body, the internal body
structures absorb varying amount of the radiation. The radiation that leaves the body has a
spatial intensity variation i.e. an image of the internal structure of the body. The commonly
used arrangement for diagnostic radiology is shown in figure.

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The X-ray intensity distribution is visualized by a suitable device like a photographic film.
A shadow image is generated that corresponds to the X-ray density of the organs in the body
section.
The main properties of X-rays, which make them suitable for the purpose of medical
diagnosis are their-
• Capability to penetrate matter coupled with different absorption observed in various
material
• Ability to produce luminescence and its effect on the photographic emulsions.
The X-ray picture is called a radiograph, which is a shadow picture produced by X-rays
emanating from a point source. The X-ray picture is usually obtained on photographic film
placed in the image plane.
Nature of X-rays
X-rays are electromagnetic radiation located at the low wavelength end of the
electromagnetic spectrum. The X-rays in the medical diagnostic region have wavelength of
the order of 10-10m. They propagate with a speed of 3 x 108 m/s and are unaffected by
electric and magnetic fields. According to the quantum theory, electromagnetic radiation
consists of photons, which are conceived as ‘Packets’ of energy. Their interaction with
matter involves an energy exchange and the relation between the wavelength and the photon
is given by,
E = νh = λ
hc
, where
h = Plank’s constant = 6.32 x 10-34
Js
c = velocity of propagation of photons
= 3 x 108
m/s
ν = frequency of radiation
λ = wavelength

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A vibration can be characterized either by its frequency or by its wavelength. In the case of
X-rays, the wavelength is directly dependent on the voltage with which the radiation is
produced.
Properties of X-rays
They travel in straight line.
Because of short wavelength and extremely high energy, X-rays are able to penetrate
through materials which readily absorb and reflect visible light.
Not deflected by electric or magnetic field.
X-rays produce ionization in gases and influence the electric properties of liquids
and solids.
Cause certain substances to fluorescence helping them to emit light , e.g. Barium
platinocyanide.
Affect a photographic emulsion in a similar manner to light.
Unit of X-ray
The International Commission on Radiological units and Measurement has adapted
Rontgen as a measure of the quantity of X-ray radiation. This unit is based on the ability
of radiation to produce ionization and is abbreviated ‘R’.
1.1.3 Principle of operation of X-ray tube
X-Rays are produced by energy conversions when fast moving electrons from the
filament of the X-Ray tube interact with the tungsten anode (target). X-rays are
generated by two different processes:-
General radiation (Bremsstrahlung)
Characteristic radiation (K-Shell)
Fig.a X-ray tube Fig.b Typical spectrum
of X-ray produced

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Electrons are emitted from the heated cathode (Thermionic effect). The electrons are
accelerated through a large potential difference (20kV to 100 kV for diagnosis) before
bombarding a metal anode. X-rays are generated when fast moving electrons are
suddenly decelerated by impinging on a target metal of high melting point and high
atomic weight (like tungsten, molybdenum). These X-rays produced leave the tube via a
‘window’. An X-ray tube is basically a high vaccum diode with a heated cathode located
opposite a target anode as shown in figure. This diode is operated in the saturated mode
with a fairly low cathode temperature so that the current through the tube does not
depend on the applied anode high voltage. Since the majority of the energy of the
electrons is transferred to thermal energy in the metal anode, the anode is either water
cooled or is made to spin rapidly so that the target area is increased. The anode is held at
earth potential.
The intensity of the X-rays depends on the current through the tube. This current can be
varied by varying the heater current which in turn controls the cathode temperature. The
wavelength of the X-rays depends on the target material and the velocity of the electrons
hitting the target. It can be varied by varying the target voltage of the tube. The electron
beam is concentrated to form a small spot on the target. The X-rays emerge in all
directions from this spot, which therefore can be considered a point source for the
radiation.
The hardness of the X-ray beam (i.e. penetration of the X-rays) is controlled by the
accelerating voltage between the cathode and anode. More penetrating X-rays have
higher photon energies and thus a larger accelerating potential is required. Referring
fig.b it can be seen that longer wavelength X-rays (‘Softer’ X-rays) are always also
produced. Indeed some X-ray photons are of such low energy that they would be not
able to pass through the patient. These ‘soft’ X-rays would contribute to the total
radiation dose without any useful purpose. Consequently, an aluminium filter is
frequently fitted across the window of the X-ray tube to absorb the soft X-ray photons.
1.1.4 Instrumentation for Diagnostic X-rays
Fig. Instrumentation for diagnostic X-ray (Chest X-ray)
The use of X-rays as a diagnostic tool is based on the fact that various anatomical structrures
of the body have different densities for the X-rays. When X-rays from a point source

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penetrate a body section, the internal structure of the body absorbs varying amount of the
radiation. The radiation that leaves the body, therefore, has a spatial intensity variation that
is an image of the internal structure of the body. When this intensity distribution is
visualized by a suitable device, a shadow image is generated that corresponds to the X-ray
density of the organs in the body section. If a picture is required of bones, this is relatively
simple since the absorption by bone of X-ray photons is considerably greater than the
absorption by surrounding muscles and tissues. X-ray pictures of other parts of the body
may be obtained if there is sufficient difference between the absorption properties of the
organ under review and the surrounding tissues.
The quality of the shadow picture produced on the photographic plate depends on its
sharpness and contrast. Sharpness is concerned with the ease with which the edges of
structures can be determined. A sharp image implies that the edges of organs are clearly
defined. An image may be sharp but, unless there is a marked difference in the degree of
blackening of the image between one organ and another, the information that can be gained
is limited. An X-ray plate with a wide range of exposures, having areas showing little or no
blackening as well as areas of heavy blackening is said to have good contrast.
In order to achieve as sharp an image as possible, the X-ray tube is designed to generate a
beam of X-rays with minimum width. Factors in the design of the X-ray instrumentation for
diagnosis that may affect sharpness include-
• the area of the target anode
• the size of the aperture, produced by overlapping metal plates, through which the X-
ray beam passes after leaving the tube
• the use of a lead grid in front of the photographic film to absorb scattering X-ray
photons
1.1.5 Introduction to X-ray Machine
X-ray Machine consists of following -
Fig. X-ray Machine
Cathode is the negative terminal of the X-Ray tube. It consists of:-
A filament-source of electrons
A Metallic Focusing cup
Glass enclosure
Cathode
Anode
Stationary
anode
Rotating anode

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Connecting wires to supply voltage and ampere to heat the filament
Filament is made up of tungsten wires(0.2mm dia) coiled to form a vertical spiral
,0.2cm in dia. and 1cm in length. Tungsten - high melting point (33700C) and
strong. Current heats the filament as a result electrons are emitted due to thermionic
emission. This process of formation of electron cloud is called Edison effect.
Anode is the positive electrode of the tube. It is of two types-
Stationary anode
Rotating anode
Stationary anode
Small plate of tungsten(2-3mm thick) embedded in large mass of copper
Square or rectangular in shape
Anode angle 15-200
Tungsten –
High atomic no.-more electrons for X-Ray production
High melting point (33700C)
Good material for absorption and dissipation of heat
Large copper portion of the anode facilitates heat dissipation as Cu is a better
conductor of heat
Rotating anode
Large disc of tungsten rotates at a speed of 3600rpm
Tungsten disc has a beveled edge- angle 6-200
Purpose-spread the heat produced during an exposure over a large area of anode
Disc diameter-75,100 or 125mm
Mechanical problems
Power to effect rotation
Friction- Lubrication
Oil vaporizes-destroy tube
Graphite wear off-destroy tube
Metallic lubricants suitable e.g. silver
Heat dissipation-absorption of heat by anode assembly is undesirable-malfunctioning
of bearings. Stem- molybdenum (bad conductor/high M.P)
Tube shielding
Lined with lead
Absorbs primary and scattered X-Rays-reduces needless exposure & film
fogging
To provide shielding for high voltages required to produce X-Rays.
To prevent short circuiting between the grounding wires & the tube the space
between them is filled with thick mineral oil.
Oil has good electrical insulating & thermal cooling properties.

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1.2 Mass Spectrometry
1.2.1 Definition, purpose and Application
Definition:
Mass spectrometry is a powerful analytical technique that is used to identify
unknown compounds(both qualitative and quantitative), to quantify known
compounds and to elucidate the structure and chemical properties of molecules.
It measures the masses of individual molecules that have been converted into ions,
i.e., molecules that have been electrically charged.
This unit of mass is often referred to by chemists and biochemists as the dalton (Da
for short), and is defined as follows: 1 Da=(1/12) of the mass of a single atom of the
isotope of carbon-12(12C).
Compounds can be identified at very low concentration (1 part in 1E12) in
chemically complex mixtures.
Purpose:
To detect and identify the use of steroid in the athelets
To monitor the breath of the anesthesiologists during surgery
To determine the composition of molecular species found in space
To determine whether honey is adulterated with corn syrup
To detect the dioxins in contaminated fish
To monitor the fermentation processes for the biotechnology industry
To determine gene damage from environmental causes
To locate oil deposits by measuring petroleum precursors in rock
To establish the elemental composition of semiconductor materials
Application:
Identify the structures of biomolecules ( carbohydrate, nucleic acid, steroid)
Sequence Biopolymers such as proteins and oligosaccharides
Pharmacokinetics is often studied using mass spectrometry because of the complex
nature of the matrix (often blood or urine) and the need for high sensitivity to
observe low dose and long time point data.
Perform Forensic analysis such as conformation and quantitation of drugs of abuse
Analyze environmental pollutants
Determining the age and origins of specimens in geochemistry and archaeology
Identify and quantitate compouds of complex organic mixtures
Perform ultrasensitive multielement inorganic analysis

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1.2.2 Simple Block Diagram
1.2.3 Principle of operation
The different functional units of a mass Spectrometer are represented conceptually in the
block diagram as shown in the figure. However, the four major steps are illustrated in brief
below which are very essential in it.
Stage 1: Ionization The atom is ionized by knocking one or more electrons off to give a
positive ion. Mass spectrometers always work with positive ions.
Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic
energy.

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Stage 3: Deflection The ions are then deflected by a magnetic field according to their
masses. The lighter they are, the more they are deflected. The more the ion is charged, the
more it gets deflected.
Stage 4: Detection The beam of ions passing through the machine is detected electrically
Fig. Principle of operation
Mass spectrometers can be divided into three fundamental parts, namely the ionisation
source , the analyser , and the detector.
The sample has to be introduced into the ionisation source of the instrument. Once inside the
ionisation source, the sample molecules are ionised, because ions are easier to manipulate
than neutral molecules. These ions are extracted into the analyser region of the mass
spectrometer where they are separated according to their mass (m) -to-charge (z) ratios
(m/z) . The separated ions are detected where the ion flux is converted to a proportional
electrical current and this signal sent to a data system where the m/z ratios are stored
together with their relative abundance for presentation in the format of a m/z spectrum .
The analyser and detector of the mass spectrometer, and often the ionisation source too, are
maintained under high vacuum to give the ions a reasonable chance of travelling from one
end of the instrument to the other without any hindrance from air molecules. The entire
operation of the mass spectrometer, and often the sample introduction process also, is under
complete data system control on modern mass spectrometers.
Simplified schematic of a mass spectrometer

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1.3 Nuclear Magnetic Resonance Instruments
1.3.1 Definition, purposes, Application areas
Definition
Nuclear magnetic resonance tomography has emerged as a powerful imaging
technique in the medical field because of its high resolution capability and potential
for chemical specific imaging.
It uses magnetic fields and radio frequency signals to obtain anatomical information
about the human body as cross sectional images in any desired direction and can be
easily discriminated between healthy and diseased tissue.
MR has much greater soft tissue contrast than Computed tomography (CT) making it
especially useful in neurological, musculoskeletal, cardiovascular and oncological
diseases
NMR images are essentially a map of the distribution density of hydrogen nuclei and
parameters reflecting their motion, in cellular water and lipids.
Application Areas
Diagnosing tumors of the pituitary gland and brain
Diagnosing infections in the brain, spine or joints
Diagnosing multiple sclerosis (MS)
Visualizing torn ligaments in the wrist, knee and ankle
Visualizing shoulder injuries
Diagnosing tendonitis
Evaluating masses in the soft tissues of the body
Evaluating bone tumors, cysts and bulging or herniated discs in the spine
Diagnosing strokes in their earliest stages
MR Angiography
Advantages of NMR imaging System
The advantages of the NMR imaging system are
It provides image between soft tissues that are nearly identical.
Cross-sectional images with any orientation are possible
NMR imaging parameters areaffected by chemical bonding and therefore offer
potential for physiological imaging
NMR uses no ionizing radiation
NMR imaging requires no moving parts, gantries or sophisticated crystal detectors.
NMR permits imaging of entire 3D volumes simultaneously instead of slice by slice.
Biological Effects of NMR imaging
The three aspects of NMR imaging which could cause potential health hazar are-
Heating due to the rf power
Static magnetic field

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Electric current induction to rapid change in magnetic field
1.3.2 Principle of operation
Many atomic nuclei behave as if they possess a ‘spin’. Such nuclei have an odd number
of protons and/or an odd number of neutrons. Their ‘spin’ causes the nuclei of these atoms
to behave as tiny magnets. If an external magnetic field is applied to these atoms, they will
tend to line up in the magnetic field. This alignment is not perfect and the nuclei rotate about
the direction of the field as they spin. This type of motion is referred to as precession. The
motion is similar to the motion of a top spinning in a gravitational field.
The frequency of precession (the Lamour frequency) depends on the nature of the
nucleus and the strength of the magnetic field. The Lamour frequency is found to lie in the
radio-frequency (RF) region of the electromagnetic spectrum.
If a short pulse of radio waves of frequency equal to the Lamour frequency is applied, the
atoms will resonate, absorbing energy. When the pulse ends, the atoms will return to their
original equilibrium state after a short period of time, called the relaxation time. In so doing,
RF radiation is emitted by the atoms. There are, in fact, two relaxation processes and it is the
times between these that forms the basis of magnetic resonance imaging (MRI).
Examples of nuclei that show this effect include hydrogen, carbon and phosphorus.
Because of its abundance in body tissue and fluids, hydrogen is the atom used in this
scanning tehnique.
A schematic diagram of a magnetic resonance (MR) scanner is shown in Fig.
The person under investigation is placed between the poles of a very large magnet that
produces a uniform magnetic field in excess of 1 tesla. All the hydrogen nuclei within the
person would have the same Lamour frequency because this frequency is dependent on the
magnetic field strength. In order to locate a particular position of hydrogen atoms within the
person, a non-uniform magnetic field is also applied. This non-uniform field is accurately
calibrated so that there is a unique value of magnetic field

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strength at each point in the person. This value, coupled with the particular value of the
Lamour frequency, enables the hydrogen nuclei to be located.
Radio-frequency pulses are transmitted to the person by means of suitable coils.
These coils are also used to detect the RF emissions from the patient. The received
emissions are processed in order to construct an image of the number density of hydrogen
atoms in the patient. As the non-uniform magnetic field is changed, then atoms in different
parts of the person will be detected.
Fig. a) Random alignment of magnetic moments of nuclei
b) Alignment in Uniform strong magnetic field B0
1.3.3 Simple Block Diagram
Basic NMR Components
Fig. Block Diagram of a NMRI System

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The basic components of an NMR imaging system are shown in figure. These are
i) A magnet, which provides a strong uniform, steady, magnetic field B0.
ii) An RF transmitter, which delivers radio-frequency magnetic field to the sample.
iii) A gradient system, which produces time varying magnetic fields of controlled spatial
non-uniformity.
iv) A detection system, which yields the output signal and
v) An imager system, including the computer, which reconstructs and displays the
images.
Detailed overview:
i) Magnet: In magnetic resonance tomography, the base field must be extremely
uniform in space and constant in time as its purpose is to align the nuclear
magnets parallel to each other in the volume to be examined.
ii) RF transmitter System: In order to activate the nuclei so that they emit a useful
signal, energy must be transmitted into the sample. This is what the transmitter
does. The system consists of an RF transmitter, RF power amplifier and RF
transmitting coils. The RF voltage is gated with the pulse envelops from the
computer interface to generate RF pulses that excite the resonance. These pulses
are amplified to levels varying from 100w to several kW depending on the
imaging method and are fed to the transmitter coil.
iii) Detection System: The function of the detection system (receiver) is to detect the
nuclear magnetization and generate an output signal for the processing by the
computer. The receiver coil usually surrounds the sample and acts as an antenna
to pick up the fluctuating nuclear magnetization of the sample and converts it to
a fluctuating output voltage v(t).
Some of the commonly available coils are-
• Body coils
• Head coils
• Surface coils
• Organ enclosing coils
iv) Gradient system for spatial coding: It is done for the spatial information. It
produces the time varying magnetic field which produces resonance with the
uniform field produced by permanent magnet.
v) Imager system: The imager system includes the computer for image processing,
display system and control console. The computer system collects the nuclear
magnetic resonant signal after A/D conversion, corrects, re-composes, displays
and store it. 2D images are typically displayed as 256x256 or 512x512 pixel
array. For 3D, personal computer requires more processing power. It uses 32 bit
machines upto 4Mb memory.

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1.4 Radiation
1.4.1 Generation of Ionizing Radiation
Fig. Modern X-ray Tube
X-rays are generated when fast moving electrons are suddenly decelerated by
impinging on target. An X-ray tube is a high vaccum diode with heated cathode
located opposite the target anode as shown in figure. This diode is operated in the
saturated mode with a fairly low cathode temperature so that the current through the
tube does not depend on the applied anode voltage.
The intensity of X-rays depends on the tube current which can be varied by
varying heater current, which in turn controls the cathode temperature. Wavelength
depends on target material and velocity of electrons hitting target. It can be varied by
varying the target voltage (30-100kV) of the tube.
Electron beam is concentrated to form a spot on the target- act as point
source for the radiation. Therapeutic X-ray equipment uses even higher radiation
energies which require linear or circular particle accelerators.
1.4.2 Nuclear Radiation
Radioactive decay is one of the source of Nuclear Radiation
Artificial radioactivity is done by exposing them to neutrons generated with a
cyclotron or in an atomic reactor
At the moment of disintegration radiation is emitted
Radioactive element has half life from few seconds to thousands of years
The radioactivity can be detected by three physical effects

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Activation on photographic emulsions
Ionization of gases
Light flashes the radiation causes when striking certain minerals
When radioactive material is introduced into human body for diagnostic purposes
radiation dose must be at a safe level. Since biological decay is slow so isotopes of
short half life must be used.
1.4.3 Instrumentation for medical use of radioisotope
Fig. Block Diagram of an instrumentation system for radioisotope
In diagnostic methods involving introduction of radioisotopes into the body / exposure for
long time – radiation intensity must be a safe dose. It is based on counting number of
nuclear disintegration. Because of random nature of radioactive decay the procedure is
afflicted with an unavoidable statistical error.
As in the block diagram of an instrumentation system for radioisotope procedures, the
sample is injected with some radioactive elements. The radioactive element has its half life
time and during which it emits the radiation. These light flashes are detected by the detector.
However, the basic steps are written in point below:-

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Most nuclear radiation detectors used for medical applications utilize the light
flashes caused by radiation ---- scintillation detectors (crystal of thallium activated
sodium iodide
Each radiation quantum passes --- causes output pulse --- proportion to energy of
radiation
Pulses from photomultiplier tube are amplified and shortened before they pass
through the pulse-height analyzer
Timer and gate allows the pulse in a set of time interval to be counted by scaler
Rate meter shows the rate of the pulses
1.4.4 Radiation Therapy
The ionizing effects of radiation are used for treatment of diseases like cancer is
called radiation therapy. In dermatology very soft X-rays that do not have enough
penetration power to enter more deeply into body are used for skin treatment. Grenz
rays (spectrum between X-ray and UV)
• In deep seated tumors, very hard X-rays are used– linear accelerators/ betatrons are
used to obtain electrons with a very high voltage
• Changing direction of entry of beam in successive therapy sessions by rotating
patients----to reduce damage to unaffected body parts
1.5 Non-invasive Techniques
1.5.1 Non- Invasive Diagnostic Instrumentation
Historically, diagnosis consisted of two techniques- observing the patient outwardly
for signs of fever, vomiting, changed breathing rate etc, and observing the patient inwardly
by surgery. The first technique depended greatly on experience but was still blind to detailed
internal conditions. The second quite often led to trauma and sometimes death of the patient.
Modern diagnostic techniques have concentrated on using externally placed devices
to obtain information from underneath the skin about the internal structures. This is
achieved without the need of investigative surgery and is described as a non-invasive
technique. Non-invasive techniques are designed to present a much smaller risk than
surgery and are, in general, far less traumatic for the patient.
1.5.2 Temperature Measurements of Body
Body temperature is one of the oldest known indicators of the general well-being of
a person. Two basic types of temperature measurements can be obtained from the human
body : systematic and skin measurements. Both provide valuable diagnostic information,
although the systematic temperature measurement is much more commonly used.
I) Systematic Temperature:

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It is the temperature of the internal regions of the body. This temperature is
maintained through a carefully controlled balance between the heat generated
by the active tissues of the body, mainly the muscles and the lever, and the
heat lost by the body to the environment. Measurement of the systematic
temperature is accomplished by the temperature sensing devices placed in the
mouth, under the armpits or in the rectum. The normal oral (mouth)
temperature of a healthy person is about 370
C (98.50
F). The underarm
temperature is about 1 degree lower, whereas the rectal temperature is about
1 degree higher than the oral reading.
II) Surface or Skin temperature:
This temperature is also a result of balance, but here the balance is between
the heat supplied by the blood circulation in a local area and the cooling of
that area by conduction, radiation, convection and evaporation. Thus, skin
temperature is a function of the surface circulation, environmental
temperature, air circulation around the area from which the measurement is
to be taken, and perspiration. To obtain a meaningful skin temperature
measurement, it is usually necessary to have the subject remain, with no
clothing covering the region of measurement in a fairly cool ambient
temperature (approx. 210
C/ 700
F).
1.5.3 Ultrasonic Measurement
Principles of ultrasonic measurement
In order to be able to explain the principles of the use of ultrasound in diagnosis, it is
necessary to have an understanding of the reflection of ultrasound at boundaries and its
absorption in media.
Ultrasound obeys the same laws of reflection and refraction at boundaries as audible sound
and light. When an ultrasound wave meets the boundary between two media, some of the
wave energy is reflected and some is transmitted, as illustrated

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For an incident intensity I, reflected intensity IR and transmitted intensity IT, then from
energy considerations,
I = IR + IT.
The relative magnitudes of the reflected and transmitted intensities depend not only on the
angle of incidence but also on the two media themselves.
When a wave is incident normally on a boundary between two media having specific
acoustic impedances of Z1 and Z2, the ratio IR / I of the reflected intensity to the incident
intensity is given by the expression,
The ratio IR / I is known as the intensity reflection coefficient for the boundary and is
usually given the symbol α. Clearly, the value of α depends on the difference between the
specific acoustic impedances of the media on each side of the boundary.
It can be seen that the intensity reflection coefficient is very large for ultrasound entering or
leaving the human body (a boundary between air and soft tissue). In order that ultrasound
waves may be transmitted from the transducer into the body (and also return to the
transducer after reflection from the boundaries of body structures), it is important to ensure
that there is no air trapped between the transducer and the skin. This is achieved by means
of a coupling medium such as a gel that fills any spaces between the transducer and the skin.
A second factor that affects the intensity of ultrasonic waves passing through a
medium is absorption. As a wave travels through a medium, energy is absorbed by the
medium and the intensity of a parallel beam decreases exponentially.
Ultrasound works in the principle of Doppler’s effect, in which the frequency of the
reflected ultrasonic energy is increased or decreased by a moving interface. The change in
frequency according to this principle is given by,
λ
v
f
2
=∆ , ∆ƒ = change in frequency;V = velocity of the interface
λ = wavelength of the transmitted ultrasound
The frequency increases when the interface moves towards the transducer and
decreases when it moves away.
Ultrasonic frequencies employed for medical applications range from 1 – 15 MHz
which are transmitted as a mechanical vibrations. The speed of ultrasound in various
biological materials is different. If the time taken by the ultrasonic wave to move from its
source through a medium, reflect from an interface and return to the source can be
measured, the depth of penetration is given by,
Depth of penetration
2
ν
≅ , v = velocity of sound in medium transmitted

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The physical mechanism normally used to generate and detect ultrasonic waves is
the piezoelectric effect exhibited by certain crystalline materials which have the property to
develop electrical potentials on definite crystal surfaces when subjected to mechanical
strain.
Piezo-electric crystals are available in several shapes and the selection of a particular
shape depends upon the application to which it is to be put. There are three parameters in
optimizing transducers for various types of applications. These are frequency, active
element diameter and focusing.
Properties of Ultrasound
1. Ultrasonic waves are sound waves associated with frequencies above the audible
range and generally extend upward from 20 kHz.
2. Transmission of ultrasonic wave motion can take place in different modes. The wave
motion may be longitudinal, transverse or shear.
3. Ultrasonic waves can be easily focused i.e. they are directional and beams can be
obtained with very little spreading.
4. They are inaudible and are suitable for applications where it is not advantageous to
employ audible frequencies.
5. By using high frequency ultrasonic waves which are associated with shorter
wavelength, it is possible to investigate the properties of very small structures.
6. Information obtained by ultrasound, particularly in dynamic studies, cannot be
acquired by any other more convenient technique.
7. Ultrasound is not only non-invasive, externally applied and non-traumatic but also
apparently safe at the acoustical intensities and duty cycle presently used in
diagnostic equipment.
Attenuation constant and Equation
As the ultrasound travels through the material some energy is absorbed and the wave
is attenuated a certain amount for each centimeter through which it travels. The amount
of attenuation is a function of both the frequency of the ultrasound and the
characteristics of the material. The attenuation constant, α, is defined by the following
equation-
β=
distanceunit1+Xpointatamplitude
Xpointatamplitude
α (per cm) = cƒβ, where c = proportionality constant
ƒ = ultrasound frequency
β = exponential term determined by the properties of
the material
This formula shows that attenuation increases with some power of frequency, which
means that the higher the frequency the less distance it can penetrate into the body with a

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given amount of ultrasonic energy. For this reason, lower ultrasound frequencies are used
for deeper penetration.
EXERCISE
Past questions
1. Explain how microprocessors can be used in Biomedical Instrumentation. (6)
2. What do you understand by infrared ray, ultraviolet ray and X-ray? Explain the
applications in brief. (8)
3. Describe the X-ray as basis of diagnostic radiology. Also mention the properties of X-
rays. (4+2)
4. Define Mass Spectrometry. What are its purpose and explain the application area of mass
spectrometry. (8)
5. With simple block diagram describe the components of mass spectrometry. (6)
6. Explain the simple block diagram of NMRI. (8)
7. What do you mean by NMRI? Explain basic NMR components along with simple block
diagram. (2+4)
8. What do you understand by Radiation Therapy? Explain in brief. (8)
9. What is the basic principle of ultrasonic measurements? Explain the properties of
ultrasound. (8)
***END OF CHAPTER-1***

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PART II- MICROPROCESS INSTRUMENTATION
CHAPTER-2
MICROPROCESSOR BASED INSTRUMENTATION
SYSTEM
Block Diagram of Instrumentation System
Fig. Functional Elements of an Instrumentation System
The output of the transducer contains information needed for further processing by
the system and the output signal is usually a voltage or some other form of electrical signals.
The signal cannot be directly transmitted to next stage without removing the interfering
sources, as otherwise highly distorted results may be obtained. It becomes necessary to
perform certain operations on the signal before it is transmitted further. These processes
may be linear like amplification, attenuation, integration, differentiation, addition and
subtraction. Some non-linear processes like modulation, detection, sampling, filtering,
chopping etc. are also performed on the signal to bring it to the desired form to be accepted
by the next stage of measurement system. This process of conversion is called signal
conditioning.
Signal conditioning or data acquisition equipment in many a situation is an
excitation and amplification system for passive transducers. It may be an amplification
system for the active transducers. The transducer output is brought up to a sufficient level to
make it useful for conversion, processing, indicating and recording.
Data Conditioning
Element

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When the elements of an instrument are physically separated, it becomes necessary
to transmit data from one to another. The element that performs this function is called data
transmission element. The signal conditioning and transmission stage is commonly known
as Intermediate stage.
The information about the quantity under measurement has to be conveyed to the
personnel handling the instrument or the system monitoring, control or analysis purpose.
2.1 Basic Components Involved in Designing a Microprocessor Based
System
A measurement system consists of-
1. Basic Functional Elements
2. Auxiliary Functional Elements
1. Basic Functional Elements: They form integral parts of all instrumentation
systems. They include-
a. Transducer Element: It senses and converts the desired input to a more
convenient and practicable form to be handled by the measurement system.
b. Signal Conditioning/ Intermediate Element: It does data manipulation or
the processing of the output of the transducer in a suitable form. It is Analog
to digital converter in microprocessor based system.
c. Data Presentation Element: In order to give the information about the
measurand or measured variable in quantitative form, data presentation is
required. Eg. Data logging, data display, data communication etc.
2. Auxiliary Functional Element: It is incorporated in a particular system depending
on the type of requirement, the nature of measurement technique etc. They include-
a. Calibration Element
b. External Power Element
c. Feedback Element
d. Microprocessor Element
Microprocessor Based System
In microprocessor based system, microprocessor forms one of the auxiliary functional
elements of the instrument. It has vast potential to perform complex computations at
fantastically high speeds, together with pre-programmed logic/ software which enhance
significantly the capabilities and effectiveness of the instruments. Microprocessor based
instruments are commonly called as smart or intelligent instruments. Some eg are pocket
size thermometer, thermocouple sensor of digital type, portable velocity meter, micro size
pressure pick ups etc.
Microprocessor itself is an operational computer. It is incorporated with additional
circuits for memory and input and output devices to shape it in the form of a digital
computer.
INPUT
DEVICES
MEMORY
CONTROL
UNIT
A.L.U
OUTPUT
DEVICES

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Fig. Digital Computer System
Input devices include keyboards, floppy diskettes, CDs, mouse, scanners etc.
Further, output devices include printers, plotters or VDU etc.
Fig. A typical block diagram of digital computer
Computer
processed
outputs
DATA LOGGING
Eg. Magnetic tape, Print out
DATA DISPLAY
Eg. VDU, X-Y plotter
DATA
COMMUNICATION
eg. Remote indication
PROCESS / PLANT/ SYSTEM
ANALOG TRANSDUCERS
MULTIPLEXERS
(To sequentially feed outputs, one at a time)
SIGNAL CONDITIONER
+
A/D
DIGITAL COMPUTER
Sequential digital o/p
Operator commands through I/O
Software

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2.2 COMPONENTS
a. OPERATIONAL AMPLIFIER CHARACTERISTICS AND CIRCUITS
Basic Op-Amp characteristics:
a) Comparator
b) Comparator with Hysteresis
The pins labeled +V and –V represent the
power-supply connections. The voltages
applied to these pins usually be +15V and -15V
or +12V and -12V. The op-amp also has two
signal inputs. The input labeled with a – sign is
called the inverting input and the input labeled
+ sign is called the non-inverting input. If the
inverting input is made more positive than the
non inverting input, the o/p signal will be
inverted or 1800
out of phase with the input
signal.
The ratio of the voltage out from an amplifier
circuit to the input voltage is called voltage
gain “Av”. The Av for an op-amp is typically
100,000 or more.
In this circuit the op-amp effectively
compares the input voltage with the
voltage on the inverting input and gives
a high or low output, depending on the
result of the comparison. If the input is
more than a few microvolts above the
reference voltage on the inverting
input, the output will be high (goes into
saturation of +12V). If the input
voltage is few microvolts more
negative than the reference voltage, the
output will be low (goes into saturation
at -12V). An op-amp used in this way
is called a comparator.
O/P = +V-1V
If Vin < Vref
O/P = -V +1V
If Vin > Vref
In this circuit reference signal is
applied to the non-inverting input and
input voltage is applied to the
inverting input. The positive feedback
resistor from the output to the non-
inverting input is applied. This
feedback gives the comparator a
characteristic called hysteresis.

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Hysteresis means that the output voltage changes at a different input voltage when
the input is going in the positive direction than it does when the input voltage is going in the
negative direction. Hysteresis prevents the noise from causing the comparator output to
oscillate as the input signal gets close to the reference voltage.
c) Non-inverting Amplifier
d) Inverting Amplifier
Vhysteresis = Vref x R1/(R1+R2)
The input signal is applied to the non-
inverting input, so the output will be in
phase with the input. A fraction of the
output signal is fed back to the inverting
input.
The voltage gain of a circuit with feedback
is called its closed-loop gain ‘AVCL’ which
is equal to simple resistor ratio.
In this circuit non-inverting input is
tied to ground with a resistor. Since,
the signal is applied to the inverting
input; the output signal will be 1800
out of phase with the input signal. For
this circuit R2 supplies the negative
feedback which keeps the two inputs
at nearly the same voltage. Since, the
non-inverting input is tied to ground;
the op amp will sink or source
whatever current is needed to hold the
inverting input also at zero volts this
node is referred to as a virtual ground.

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e) Differential Amplifier
f) Summing/ Adder Amplifier
Here, the resistors on the non-
inverting input hold this input at a
voltage near the common mode dc
voltage. The amplifier holds the
inverting input at the same voltage.
If the resistors are matched carefully,
the result is that only the difference
in voltage between V2 and V1 will
be amplified. The output signal will
consist of only the amplified
difference in voltage between the
input signals.
In fig. input voltage V1 produces a
current through R1 to this point and
input voltage V2 causes a current
through R2 and similar for V3. The
three currents add together at the
virtual ground. The virtual ground is
often called the summing point. The
op amp pulls the sum of two currents
through resistor R4 to hold the
inverting input at 0V. The left end of
R4 is at 0V, so the output voltage is
the voltage across R4. This to the sum
of the current times the value of R4

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g) Integrator
h) Differentiator
An op-amp circuit shown can be used to
produce linear voltage ramps. A dc voltage
applied to input of this circuit will cause a
constant current of VIN/R1 to flow into the
virtual-ground point. This current flows onto
one plate of the capacitor. In order to hold the
inverting input at ground, the op-amp output
must pull the same current from the other plate
of the capacitor. The capacitor then is getting
charged by the constant current VIN/ R1 which
gives a linear ramp. Since, the inverting
amplifier connection, a positive voltage will
cause the output to ramp negative.
The circuit is called an integrator because it
produces an output voltage proportional to the
integral of the current produced by an output
voltage over a period of time.
The right-hand side of the capacitor is held to a voltage of 0 volts, due to the "virtual ground"
effect. Therefore, current "through" the capacitor is solely due to change in the input voltage. A
steady input voltage won't cause a current through C, but a changing input voltage will. Capacitor
current moves through the feedback resistor, producing a drop across it, which is the same as the
output voltage. A linear, positive rate of input voltage change will result in a steady negative
voltage at the output of the op-amp. Conversely, a linear, negative rate of input voltage change
will result in a steady positive voltage at the output of the op-amp. This polarity inversion from
input to output is due to the fact that the input signal is being sent (essentially) to the inverting
input of the op-amp, so it acts like the inverting amplifier mentioned previously. The faster the rate
of voltage change at the input (either positive or negative), the greater the voltage at the output.

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g) Instrumentation Amplifier
SIGNAL GAIN
The instrumentation amp offers two useful functions: amplify the difference between inputs
and reject the signal that’s common to the inputs. The latter is called Common Mode
Rejection (CMR). The signal gain is accomplished by XOP1 and XOP2 while XOP3
typically forms a differential gain of 1. You can calculate the overall gain by
Where, R1=R3 and R5/R4 = R7/R6.
b) Sensors and Transducers
Light Sensors
Fig. shows an op-amp circuit used in
applications that need a greater rejection of
common mode signal than is provided by
the simple differential circuit. The first two
op-amps in this circuit buffer the differential
signals and give some amplification. The
output op-amp removes the common mode
voltage and provides further amplification.
One of simple light sensors is a light dependent resistor(LDR) such as
Clairex CL905 shown in figure. A glass window allows light to fall on a
zigzag pattern of cadmium sulfide or cadmium selenide whose resistance
depends on the amount of light present. The resistance of the CL905 varies
from about 15Mῼ ῼwhen in the dark to about 15K when in a bright light.

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Simple Light Controller relay circuit using a photocell
Fig. Light-controller relay circuit using a photocell
Above figure shows the circuit diagram containing a photoresistor, a
transistor driver (darlington) and a mechanical relay. When it gets dark, the resistance of the
photoresistor goes high. This increases to voltage on the base of the transistor until, at some
point, it turns on. This turns on the transistor driving the relay, which in turn turn swicthes
on the lamp.
In fig. BC547 (npn transistor) is used in darlington mode to switch the
200mA relay coil. As, the saturation of BC547 is ~ 100mA so inorder to increase effective
hfe (~ 1000) we use the darlington pair. A freewheeling diode which is essential when
driving any sort of inductive load is used. A small capacitor (0.1µF) is also placed to help
absorb the initial back EMF spike generated, as diode cannot turn instantaneously.
Principle of operation of photodiode circuitry to measure light
intensity:
If light is allowed to fall on junction, the reverse leakage current of the diode
increases linearly as the amount of light falling on it increases. The circuit shown can be
used to convert this small leakage current to a proportional voltage. In this circuit a negative
reference voltage is applied to the non-inverting i/p of the op-amp. The op-amp will produce
this same voltage on its inverting input, reverse biasing the photodiode. The op-amp pulls
leakage current through Rf to produce proportional voltage on the output of the amplifier.

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Example 1: A photodiode circuit such as this might be used to determine the amount of
smoke being emitted from a smokestack. Since smoke absorbs light, the amount of light
arriving at the photodetector is a measure of the amount of smoke present. An infrared LED
is used here because the photodiode is most sensitive to light wavelengths in the infrared
region.
#Solar Cell: Solar cells are large, very heavily doped silicon PN junctions. Light shining on
the solar cell causes a reverse current to flow, just as in the photodiode. Because of the large
area and heavy doping in the solar cell, however, the current produced is milliamperes rather
than microamperes. The cell functions as a light powered battery. Solar cells can be
connected in a series-parallel array to produce a solar power supply.

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c.Temperature Sensors
Temperature sensor perceives the real world environment temperature and gives the
corresponding analog voltage or current.
The four types pf temperature sensors are-
I. Semiconductor Temperature Sensors
II. Thermocouples
III. RTDs (Resistance Temperature Detectors)
IV. Thermistors
I. Semiconductor Temperature Sensors
The two main types of semiconductor temperature sensors are-
a. Temperature sensitive voltage sources and
b. Temperature sensitive current sources.
Characteristics include
• moderate temperature range (up to about 150°C),
• low cost, excellent linearity, and additional features like signal
conditioning, comparators, and digital interfaces.
eg. LM35, AD590 (discussed in detail)
a. Temperature sensitive voltage sources: An example of this type is the National
LM35. Circuit connection is shown in fig a. The voltage output from this circuit
increases by 10mV for each degree Celsius that its temperature is increased. The
LM35 does not require any external calibration or trimming to provide typical
accuracies of ±1⁄4˚C at room temperature and ±3⁄4˚C over a full −55 to +150˚C
temperature range. The output is adjusted to 0V for 00
C.
Fig. (a) LM35 temperature dependent voltage source
b. Temperature sensitive current sources: AD590 produces a current of 1µA/K.
Figure b shows a circuit which converts this current to a proportional voltage. In
this circuit the current from the sensor, IT, is passed through an approximately
1KΩ resistor to ground. This produces a voltage which changes by 1mV/K. The
AD580 in the circuit is a precision voltage reference used to produce a reference
Vout = +1500mV AT 1500
C
+250 mV at 250
C
-550 mV at -550
C
A
V
R s
f
µ50
−
=

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BY: Er. Sharib Ali
voltage of 273.3 mV. With this voltage applied to the inverting input of the
amplifier, the amplifier output will be zero volts for 00
C. The advantage of a
current source sensor is that voltage drops in long connecting wires do not have
any effect on the output value. If the gain and offset are carefully adjusted, the
accuracy of the circuit is ± 10
C.
Fig(b). AD590 temperature dependent current source
II. Thermocouples: Whenever two different metals are put in contact, a small voltage
is produced between them. The voltage developed depends on the type of metals
used and the temperature. Depending on the metals, the developed voltage
increases between 7 and 25 µV for each degree Celsius increase in temperature.
Different combinations of metals are useful for measuring different temperature
ranges. A thermocouple junction made of iron and constantan, commonly called
a type J thermocouple, has a useful temperature range of about -184 to +7600
C.
A junction of platinum and an alloy of platinum and 13 % rhodium has a range
of 0 to about 16000
C. Thermocouples can be made small, rugged and stable.
However, they have three major problems which must be overcome.
i. the output is very small and must be amplified before the signal
conversion
ii. a reference junction made of same metals must be connected in series
in the reverse direction as in fig. This is done so that the output
connecting wires are both constantan. The thermocouples formed by
connecting these wires to the copper wires going to the amplifierwill
then cancel out. The input voltage to the amplifier will be difference
between the voltages across the two thermocouples. If we simply
amplify this voltage, there is problem if the temperature of both
thermocouples is changing.Thus, it is impossible to tell which
thermocouple caused the change in the output voltage.
iii. The third problem is that their output voltages do not change linearly
with temperature. This can however be corrected with analog
Offset Reference
+2.5V
Instrumentation
Amplifier Gain of
10, 0.00 V to 1.00 FS
10mV/0
C
1 KΩ 0.1% LOW
TCR METERING
RESISTOR,
1mV/µA = 1mV/K
Remote Temperature
to current transducer,
1µA/K
Measured temp 0 to
1000
C

INSTRUMENTATION-II
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BY: Er. Sharib Ali
circuitry which changes the gain of an amplifier according to the
value of the signal.
• wide temperature range (up to 1250°C),
• low-cost, very low output voltage (on the order of 40µV per °C for
type K),
• reasonable linearity, and moderately complex signal conditioning
(cold-junction compensation and amplification). Eg, J type(to be
discussed), K-type
Fig©. Circuit showing amplification and cold junction
Compensation for thermocouple
III.RTDS: Resistance temperature detectors are resistors which change value with a
change in temperature. RTDs consists of a wire or a thin film platinum or a
nickel wire. The response of RTDs is nonlinear, but they have excellent stability
and repeatability. Therefore, they are often used in applications where very
precise temperature measurement is needed. RTDs are useful for measures in the
range of -250 to +8500
C.
• wide temperature range (up to 750°C), excellent accuracy and
repeatability,
• Reasonable linearity, and the need for signal conditioning.
• Signal conditioning for an RTD usually consists of a precision
current source and a high-resolution ADC.
150
C<Ta<350
C
R
E0 = Vt-Va+
R
VIa
Ω
+
+Ω
3.52
1
5.23.52
- 2.5V ≈ Vt
Va

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A circuit diagram such as that in fig. d can be used to convert the change in
resistance of the RTD to a proportional voltage. Op-amp A1 in the circuit
produces a precise reference voltage of -6.25V. This voltage produces a precise
current at inverting input of A2. Op-amp A2 pulls this current through the RTD
to produce a voltage proportional to the resistance of the RTD. The resistance of
an RTD increases with an increase in temperature.
Fig.(d) 100Ω RTD connected to perform temperature
Measurements in the range 00
C to 2660
C.
IV.Thermistors: They consist of semiconductor material whose resistance decreases
nonlinearly with temperature. They are relatively inexpensive, have very fast
response times, and are useful in applications where precise measurement is not
required.
• moderate temperature range (up to 150°C),
• low-to-moderate cost (depending on accuracy),
• poor but predictable linearity, and the need for some signal
conditioning.
A circuit similar to fig.d can be used to produce a voltage proportional to the
resistance of the thermistor.
Eout
0 TO 1.8v
FOR 0 TO
2660
C
Vref

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d.Force and Pressure
Objective: Principle of operation: Introduction to strain gages and load cells,
simple circuit connection diagram; Uses of Strain gages in various forms; Introduction
of LVDTs with simple structure diagram
STRAIN GAGES AND LOAD CELLS
They change resistance at their output terminals when stretched or compressed. It
may be made of thin wire, thin foil or semiconductor material. Because of this
characteristic, the gages are typically bonded to the surface of a solid material and measure
its minute dimensional changes when put in compression or tension. Strain gages and strain
gage principles are often used in devices for measuring acceleration, pressure, tension, and
force. Strain is a dimensionless unit, defined as a change in length per unit length. For
example, if a 1-m long bar stretches to 1.000002 m, the strain is defined as 2 microstrains.
Strain gages have a characteristic gage factor, defined as the fractional change in resistance
divided by the strain.
Figure (a) shows a simple setup for measuring force or weight. A strain gage is glued
on the top of the flexible bar. The force or weight to be measured is applied to the
unattached end of the bar. The applied force bends the bar, the strain gage is stretched,
increasing its resistance. Since the amount that the bar is bent is directly proportional to the
applied force, the change in resistance(∆R) will be proportional to the applied force. If a
current is passed through the strain gage, then the change in voltage across the strain gage
will be proportional to the applied force.
STRAIN GAGES
Fig.(a) Strain gages used to measure force
Limitation: ∆R changes with temperature
Removal: Two strain gage elements are mounted at right angles. In doing so, both change
resistance with temperature but only one will change R with the applied force.
Connection in balanced bridge
Here in balanced bridge circuit as in figure (b), any change in resistance (∆R) due to
temperature will not cause change in differential output of the bridge. Thus only the force
applied cause change in the resistance and produce a small differential output voltage.
WEIGTH

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Fig(b) Circuit connection
Pressure Measurement:
A strain gage mounted on a movable diaphragm in a threaded housing gives a output
proportional to pressure applied to the diaphragm.
If vaccum on one side of diaphragm then it gives measure of absolute pressure and
if open on one side of the diaphragm then it gives measure of atmospheric pressure.
If two sides of the diaphragm are connected to two different pressure sources, then
the output will be a measure of the differential pressure between the two sides.
LINEAR VARIABLE DIFFERENTIAL TRANSFORMERS
A linear variable differential transformer (LVDT) is a position sensor, consisting of
a primary coil, two secondary coils and a displaceable iron core (Figure 1). An AC voltage
is supplied to the primary coil. The two secondary coils are connected in anti-series. When
the core is positioned in the centre of the coil arrangement, the two secondary signals have
the same magnitude, so the output from the anti-serial connection is zero.
If the core is displaced from the centre position, one secondary produces a higher
output voltage and the other a lower output voltage. The output from the anti-serial
connection will thus increase proportional to the displacement from the centre position.
Fig.(c) LVDT s
100K
Ω
Ω

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If a spring is added so that a force is required to move the core, then the voltage out of the
LVDT will be proportional to the force applied to the core. Similarly, if a spring is added
and the core is attached to a diaphragm in a threaded housing, the output from the LVDT
will be proportional to the pressure exerted on the diaphragm.
e. Flow Sensors
Objective: Principle of operation- Types of flow sensor;
4to 20 mA current loop
a) Paddle Wheel
The rate at which paddle wheel turns is proportional to the rate of the
flow of liquid or gas. An optical encoder can be attached to the shaft of
the paddle wheel to produce digital information as to how fast the paddle
wheel is turning.
b) Differential Pressure Transducer
It gives an output proportional to the difference in pressure between the
two sides of the resistance.
4 to 20 mA Current loop
In industrial application, where sensor is at a long distance from
ADC’s the signals from sensors are converted to current instead of voltages.
In doing so, the signal amplitude is not affected by resistance or induced
voltage or voltage drops.
4mA represents open circuit i.e. 0 output while 20mA represents the
full-scale value. At receiving end, a resistor or op-amp is used to convert it
into useful voltage to be applied to the input of the A/D converter.
FLOW
Resistance
Differential Pressure
Transducer

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2.3 Interfacing Between Analog Devices
a.Use of analog devices
In order to control the machines in our electronics industries, medical instruments or
automobiles with microprocessor, we have to determine the values of variables such as
pressure, temperature and flow. There are many ways to get electrical signals which
represent the values of these variables and converting electrical signals to digital forms
the microcomputer can work with.
When we talk of analog device, the first step of design involves a sensor, which
converts the physical variables like pressure, temperature, humidity etc. to a proportional
voltage or current. We know the electrical signals from the sensors are quite small, so
they must next be amplified and perhaps filtered. This is generally done with the help of
op-amp circuit. The final step that we can use is to convert to the digital form which can
be done by an analog to digital (A/D) converter. So, here we will discuss the basic
operations, interfacing with the microprocessor of both A/D converters and D/A
converters.
While talking of the interfacing we must also deal with 8255A.
b.Operation of Comparator
The most elementary form of communication between the analog and digital device
is a device called comparator. This device is shown schematically below, which compares
two analog voltages on its input terminals. Depending on which voltage is larger, the output
will be a 1(high) or a 0(low) digital signal.
The comparator is extensively used for alarm signals to computers or digital
processing systems. This element is also an integral part of the analog to digital and digital
to analog converter.
One of the voltages on the comparator inputs, Va or Vb will be the variable input, and the
other a fixed value called a trip, trigger or reference voltage. The reference value is
computed from the specifications of the problem and then applied to the appropriate
comparator input terminal as shown below;
Fig.
Va
Vb
1 if Vb > Va
0 if Vb< Va
2.2mV/0
CTemp. (T)
0.2V/kPaPresure(P)

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The reference voltage may be provided from a divider using available power supplies.
Example: A process control system specifies that the temperature should never exceed
10Kpa. Design an alarm system to detect this condition, using temperature and pressure
transducers with the transfer functions of 2.2mV/0
C and 0.2V/kPa respectively.
Solution:
Alarm condition will be a temperature of 1600
C which gives 2.2mV/0
C × 1600
C=0.352V
which will be coincident with a pressure signal of 0.2V/kPA × 10kPa=2.0V
So, the above circuit shows how this alarm can be implemented with comparators and one
AND gate. The reference voltage could be provided from dividers.
c.Open –Collector Comparator
If the output terminal of the comparator is connected internally to the collector of a
transistor in a comparator and nowhere else, then it is called open collector comparator.
Even if there is base emitter current in the transistor, no voltage will show up on the
collector until it is connected to a supply through some collector resistor. It is shown below-
The following figure below shows that an external resistor is connected from the output to
an appropriate power supply. This is called a collector pull-up resistor.
Now, the output terminal will show either a 0 (0V) if the internal transistor is ON or 1 (+Vs)
if the internal transistor is OFF.

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Advantages
I. It is possible to use a different power supply source for the output. For example,
suppose you want to activate a +12V relay with the output of a comparator that
operates on +5V. By using an open collector model, you can connect the pull-up
resistor to a +12V supply and power the relay directly from the output.
II. It is possible to or together several comparators outputs by connecting all open
collector outputs together and then using a common pull-up resistor. If any one of
the comparator’s output transistor is turned ON, the common output will go
LOW.
Hysteresis Comparator
To solve the problem of voltage fluctuations, it is used.
Feedback resistor Rf is provided between the output and one of the inputs of the comparator,
and that input is separated from the signal by another resistor R. Under the condition that
Rf>>R, the response of the comparator is shown above. Te condition for which the output
will go high(V0) is defined by,
Vin ≥ Vref
Once having been driven high, the condition for which the output to drop back to low(0V)
state is given by the relation
Vin≤ Vref -
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
fR
R
V0
Where, R/Rf × V0 is called dead band or hysteresis
Example
A sensor converts the liquid level in a tank to voltage according to the transfer function
(20mV/cm). A comparator is supplied to go high (5V0 whenever the level becomes 50cm.
Splashing causes the level to fluctuate by ±3cm. Develop a hysteresis comparator to protect
against the effect of splashing.

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BY: Er. Sharib Ali
Nominal reference voltage for comparator at 50cm; Vref = (20mV/cm) × 50cm=1V
Splashing causes “noise” of (20mV/cm) × (±3cm) = ±60mV
This is a total range of 120mV
So, we need a deadband of 120mV, but let us make it 150mV for security.
Thus,
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
fR
R
×(5V) = 150mV
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
fR
R
= 0.03
If we make Rf = 100KΩ, then R = 3KΩ. Thus, the use of these resistors with ref. voltage of
1V will meet the above requirement.
c) DIGITAL TO ANALOG CONVERTER(DAC)
Objective: The digital-to-analog converter, known as the D/A converter (read as D-to-A
Converter) or the DAC is a major interface circuit that forms the bridge between the analog
and digital worlds. DACs are the core of many circuits and instruments, including digital
voltmeters, plotters, oscilloscope displays, and many computer-controlled devices. Here we
will examine the basis of digital-to-analog conversion, interfacing, and how it is used for
waveform generation.
What is a DAC?
A DAC is an electronic component that converts digital logic levels into an analog voltage.
When DAC is used in connection with a computer, this binary number or digital input is
called a binary word or computer word. The digits are called bits of the word.
Convert
Signal
Fig. Block Diagram of basic digital-to-analog converter (DAC)
There are basically two types of DAC:
1. Unipolar DAC: It converts a digital word into an analog voltage by scaling the
analog output to be zero when all bits are zero and some maximum value when all
RESISTIVE
SUMMING
NETWORK
Digital
Input
Reference
Voltage
Source
VOLTAGE
SWITCHES Amplifier
R
E
G
I
S
T
E
R
Analog
output
RESISTIVE
SUMMING
NETWORK

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BY: Er. Sharib Ali
bits are one. This can be mathematically represented by treating the binary number
that the word represents as a fractional number.
The output of DAC is defined as
Vout = VR [ b12-1
+ b22-2
+b32-3
+----------+bn2-n
]
Where, Vout = analog voltage ouput
VR = reference voltage
B1, b2, b3…….bn = n-bit binary word
The output of a DAC is just the sum of all the input bits weighted in a particular
manner:
Where, wi is a weighting factor, bi is the bit value (1 or 0), and i is the index of the
bit number. In the case of a binary weighting scheme, wi = 2
i
, the complete expression for
an 8-bit DAC is written as,
DAC = 128 b7 + 64 b6 + 32 b5 + 16 b4 + 8 b3 + 4 b2 + 2 b1+ 1 b0
So,
Vout= VR[128 b7 + 64 b6 + 32 b5 + 16 b4 + 8 b3 + 4 b2 + 2 b1+ 1 b0]
Now, maximum voltage for 8-bit word is thus when b0b1b2b3b4b5b6b7 = 1111111
(Vout)max = 0.9961 VR
Alternatively,
Vout = Rn
V
N
×
2
, where N = base 10 whole number equivalent of DAC input
Fig.4-bit DAC
Relationship formulas for o/p voltage with i/p binary word

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N(decimal number) = bn2n
+ bn-12n-1
+ ….............+ b121
+ b020
N = (2n
-1) , when all bits are high, n refer to no. of bits
Weight of MSB = ½ Vref
Weight of LSB = 1/2n
Vref
Vout = VR [ b12-1
+ b22-2
+b32-3
+----------+bn2-n
]
Vout = Rn
V
N
×
2
,
Conversion Resolution(∆Vout) is the increament in each step of bit combination
(∆Vout) = n
refV
2
Full scale voltage (Vfs)
Vfs = )12(
2 1
−−
nf
n
ref
R
RV
P.U. 2006. A 6-bit DAC has an i/p of (100101)2 and uses a 10.0 V reference. Then,
a) Find the o/p voltage produced
b) specify the conversion resolution
c) the DAC must have 8.00 V o/p when all input are high find the reference voltage.
[3+3+4]
Solution:
a) n = 6, Vref = 10.0 V
N= 1×25
+0 ×24
+0×23
+1×22
+0×21
+1×20
= 37,
Vout = Rn
V
N
×
2
= 6
2
37
× 10 = 5.78125V
b) ∆Vout = 10/26
= 0.15625 V
c) When all bits high N = 2n
-1 = 26
-1 = 63
6
2
63
8 =∴ ×Vref
i.e. Vref = 8.12698V
2. Bipolar DAC : It is designed to output a voltage that ranges from plus to minus
maximum value when the input binary ranges over the counting stated. Although
computer frequently uses 2’s complement to represent negative number, this is not
common with the DAC. Instead, a simple offset binary is frequently used, wherein
output is simply biased by half the reference voltage. So, the bipolar DAC
relationship is given by,

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BY: Er. Sharib Ali
Vout = Rn
V
N
×
2
-
2
RV
If N=0, Vout will be minimum value, Vout(min)= - RV
2
1
However, the maximum value for N is equal to (2n
-1), so that the max. value of
output voltage will be
Vout(max) = RRn
n
VV
2
1
2
12
−
−
i.e. Vout(max) = n
RV
2
V
2
1
R −
Example: A bipolar DAC has 10 bits and a reference of 5V. What outputs will result
from inputs of 04FH and 2A4H? What digital input gives a zero output voltage?
Solution:
04FH and 2A4H converted to base 10 numbers (79)10 and (676)10
From equation,
Vout = Rn
V
N
×
2
-
2
RV
Or, Vout = 5
1024
79
× -
2
5
= - 2.1142578V
Vout = 5
1024
676
× -
2
5 = 0.80078V
The zero occurs when the equation equals zero. Solving for N gives,
0 = 5
10
×
N
-
2
5
N = (512)12 = 200H = (10000000000)2
`DAC CHARACTERISTICS
I. Digital Input: Digital input is a parallel binary word formed by a number of bits
specified by the device specification sheet. Generally TTL logic levels are required.
II. Power supply: The power supply is bipolar at the level of ±12 to ±18V as required
for internal amplifiers. Some DACs operate from a single supply.
III. Reference Supply: A reference supply is required to establish the range of output
voltage and resolution of converter. This must be a stable, low ripple source. In some
units an internal reference is provided.
IV. Output: The output is a voltage representing the digital input. This voltage changes
in steps as the digital input changes by bits, with the steps determined by the
resolution equation.

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V. Offset: The DAC is usually implemented with op-amps so there may be the typical
output offset voltage with a zero input. Typically, connections will be provided to
facilitate a zeroing of DAC output with a zero input word.
VI. Data Latch: Many DACs have a data latch built into their inputs. When a logic
command is given to latch data, whatever data are on the input bus will be latched
into the DAC, and the analog output will be updated for that input data. The output
will stay at that value until new digital data are latched into the input. In this way,
the input of the DAC can be connected directly onto the databus of a computer , but
it will be updated only when a latch command is given by the computer.
VII. Conversion Time: A DAC performs the conversion of digital input to analog output
virtually instantaneously. From the moment that the digital signal is placed on the
output voltage is simply the propagation time of the signal through the internal
amplifiers. Typically, settling time of the internal amplifiers will be a few
microseconds.
INTERFACING AN 8 BIT DAC WITH 8085 MICROPROCESSOR
Problems Specifications
I. Design an output port with the address FFH to interface the 1408 DAC that is
calibrated for a 0 to 10 V range
II. Write a program to generate a continous ramp waveform
III. Explain the operation of the 1408 which is calibrated for a abipolar range ±5V.
Calculate the output Vout if the input is (10000000)2
I. Hardware Description:
a. The address lines are decoded by using an 8-input NAND gate. When
address lines A7 – A0 are high (FFH), the output of NAND gate goes low and
is combined with the control signal IOW in NOR gate (negative AND)
b. The digital component 74LS373 is used as a latch.
c. Industry standard 1408 DAC is used.
d. Operational amplifier 741.
e. 8085 µP with power supply and other accessories are used.
Fig: Interfacing Circuit in Unipolar Range
5K
2.5K
2.5

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The Motorola MC1408 is an 8-bit digital-to-analog converter that provides an output
current, i, directly proportional to the digital input. The transfer function found in the DAC
specifications is;
i = K {A1/2+A2/4+A3/8+A4/16+A5/32+A6/64+A7/128+A8/256}
where, the digital inputs Ai = 0 or 1, and here A1 is the most significant bit.
A8 is the least significant bit, and
the proportionality constant ,K = Vref / R14.
The reference voltage taken here as +5 V supplies a reference current of 5 V/2.5 kΩ, which
equals 2 ma through the resister R14.
The maximum current produced when all input bits are high is
0.996 * 2 ma = 1.992 ma
The output voltage Vout for full scale is ;
Vo = 2mA × (255/256) × Rf = 1.992 × 5 K = 9.961 V
II. Program to generate a continuous waveform
NOTE:
O/p’s 00 to FF continuously to DAC
Analog o/p DAC starts at 0 to 10V as ramp
When accumulator content go to 0 next cycle begins
Delay required because –a) µP needs time to execute an o/p loop which is
less than the settling time of DAC b) slope of ramp can be varied by
changing the delay
CODING
MVI A, 00H
DATAO: OUT FFH ; O/P DATA TO DAC
MVI B, COUNT ; SETUP REGISTER B FOR DELAY
DELAY: DCR B
JNZ DELAY
INR A ; LOAD NEXT I/P
JMP DTAO ; GO BACK TO O/P
III. Operating DAC in a Bipolar Range

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1408 DAC is calibrated for bipolar range ±5V by adding a resistor RB (5K) between
the reference voltage VRef and the output pin 4. The resistor RB supplies 1mA (
Vref/RB) current to the output in opposite direction of current generated by input
signal, so
I0
1
= I0 –
B
ref
R
V
=K{A1/2+A2/4+A3/8+A4/16+A5/32+A6/64+A7/128+A8/256}-
B
ref
R
V
When input signal is zero,
Vo = I0
1
Rf = (I0-
B
ref
R
V
) Rr = (0-
K5
5
) × 5K
= -5V
When input is (10000000)2
Vout = Rn
V
N
×
2
-
2
RV
= 5
2
128
8
× -
2
5
= 0V
Microprocessor Compatible DACs
The AD558 consists of four major functional blocks, fabricated on a single monolithic chip
(see Figure 2). The main D-to-A converter section uses eight equally-weighted laser-
trimmed current sources switched into a silicon-chromium thin-film R/2R resistor ladder
network to give a direct but unbuffered 0 mV to 400 mV output range. The transistors that
form the DAC switches are PNPs; this allows direct positive-voltage logic
interface and a zero-based output range.
5K
5K
2.5K
2.5K

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The high speed output buffer amplifier is operated in the noninverting mode with gain
determined by the user-connections at the output range select pin. The gain-setting
application resistors are thin-film laser-trimmed to match and track the DAC resistors and to
assure precise initial calibration of the two output ranges, 0 V to 2.56 V and 0 V to 10 V.
The amplifier output stage is an NPN transistor with passive pull-down for zero-based
output capability with a single power supply. The internal precision voltage reference is of
the patented bandgap type. This design produces a reference voltage of 1.2 volts and thus,
unlike 6.3 volt temperature compensated Zeners, may be operated from a single, low voltage
logic power supply. The microprocessor interface logic consists of an 8-bit data latch and
control circuitry. Low power, small geometry and high speed are advantages of the I2L
design as applied to this section. I2L is bipolar process compatible so that the performance
of the analog sections need not be compromised to provide on-chip logic capabilities. The
control logic allows the latches to be operated from a decoded microprocessor address and
write signal. If the application does not involve a mP or data bus, wiring CS and CE to
ground renders the latches “transparent” for direct DAC access.
TIMING AND CONTROL
The AD558 has data input latches that simplify interface to 8- and 16-bit data buses. These
latches are controlled by Chip Enable (CE) and Chip Select (CS) inputs. CE and CS are
internally “NORed” so that the latches transmit input data to the DAC section when both CE
and CS are at Logic “0”. If the application does not involve a data bus, a “00” condition
allows for direct operation of the DAC. When either CE or CS go to Logic “1”, the input
data is latched into the registers and held until both CE and CS return to “0”. (Unused CE or
CS inputs should be tied to ground.) The truth table is given in Table I.
The logic function is also shown in Figure 6.

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d. ANALOG TO DIGITAL CONVERTERS (ADC’S)
The ADCs are a quantizing process whereby an analog signal is represented by equivalent
binary states. This is opposite to DAC process. ADCs can be classified into 2 general groups
based on the conversion methods- one method involves comparing a given analog signal

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with the internally generated equivalent signal. This group encloses successive
approximation, counters and flash type converters.
The second method involves changing analog signal into time or frequency and compairing
these new parameters to known values. This group includes integrator, converters and
voltage to frequency converters.
The tradeoff between these two methods is based on accuracy Vs speed. The successive
approximation and flash method type are faster but generally less accurate than the
integrator and the voltage to frequency type converters.
The output binary number from ADC is given by,
b12-1
+ b22-2
+…………+bn2-n
≤
R
in
V
V
where, b1,b2, ……,bn = n –bit digital output
Vin= analog input voltage
VR = analog reference voltage
We use an inequality because the fraction on the right side can change continuously overall
values. But, the fraction derived from the binary number on the left can change only in fixe
increments of ∆n = 2-n
. In other words, the only way the left side can change is if the LSB
changes from 1 to 0 or from 0 to 1. In either case, the fraction changes by only 2-n
and
nothing in between.
So, there is an inherent uncertainty in the input voltage producing a given ADC output and
that uncertainty is given by
∆V=VR 2-n
Minimum and maximum Voltages:
The equation for ADC; b12-1
+b22-2
+…………+bn2-n
≤
R
in
V
V
Shows that if the ratio of input voltage to reference is less than ∆V then the digital o/p will
be all 0s, (i.e. 00000…..000)2
R
in
V
V
< ∆V
The LSB will not change until ip voltages becomes atleast equal to ∆V. (Vin≠∆V), then the
output will be (00000…..0001)2.
If the ADC o/p is all 0s and Vin<VR2-n
, it could be even be a negative voltage.
If MSB changes from 0 to 1 than the input voltage becomes equal or greater than (VR-∆V).
Therefore if the ADC o/p is all 1’s (1111……1111)2 then Vin is greater than (VR- VR2-n
).
Successive Approximation ADCs
The function of the successive approximation register, or SAR, is to make a digital estimate
of the analog input based on the 1-bit output of the comparator. The current SAR estimate is
then converted back to analog by the DAC and compared with the input. The cycle repeats
until the best" estimate is achieved. When that occurs, this present best estimate is latched

INSTRUMENTATION-II
59
BY: Er. Sharib Ali
into the output register (written into memory). By far the most common algorithm employed
by SARs is the binary search algorithm.
Comparator 311 is used to compare the input voltage, Vx, to a feedback voltage, VF, that
comes from DAC as shown above. The comparator output signal drives a logic network that
steps the digital output (and hence DAC input) until the comparator indicates the two signals
are the same within the resolution of the converter. The most common parallel feedback
converter is successive approximation registers (SAR) device. The logic circuitry is such
that it successively sets and resets each bit, starting with the most significant bit of the word.
We start with all bits zero. Thus, the first operation will be to set b1=1 and test VF = VR2-1
against Vx through the comparator. If Vx > VR2-1
then b1 will be 1, b2 is set to 1 and a test is
made for Vx versus VF = VR(2-1
+2-2
) and so on. If Vx is less than VF ( VR2-1
) then b1 is reset
to zero, b2 is set to 1 and a test is made for Vx versus VR(0×2-1
+1×2-2
). This process is
repeated to the LSB of the word.
Example: (PU) Find the successive approximation of Analog to Digital output for 5 bit ADC
to a 3.127 V input if the reference is 5V. [11]
Solution:
Here, n=5, VR = 5V, Vin= 3.127V, b4b3b2b1b0- MSB(b4) and LSB is (b0)
Step 1: Set b4 (MSB) =1
Input will be 10000. So, output of DAC (VF) = VR 2-1
= 5/2 = 2.5V
Vin>VF, so keep b4=1
Step 2: Set b3 =1
Input will be 11000. So, output of DAC (VF) = VR2-1
+ VR 2-2
= 5/2 + 5/4
= 2.5+1.25
= 3.75V
Vin < VF so clear, b3=0
Step 3: Set next bit, b2=1
Vx
VF

INSTRUMENTATION-II
60
BY: Er. Sharib Ali
Input will be 10100. So, output of DAC (VF) =VR2-1
+0+ VR2-3
=2.5 +0.625 = 3.125V
Vin>VF so keep, b2=1
Step 4: Set next bit, b1=1
Input will be 10110, output of DAC (VF) = VR2-1
+0+VR2-3
+ VR2-4
= 2.5 + 0 + 0.625+0.3125 = 3.4375V
Step 5: Set LSB b0=1
Input will be 10101, output of DAC (VF) = VR [1×2-1
+0+1×2-3
+ 0×2-4
+1×2-5
]
= 2.5+0+0.625+0+0.15625
= 3.28125
Thus, the required output is (10100)2
Alternative Method:
N = INT ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
× n
R
in
V
V
2
The base 10 number N is then converted to HEX and /or binary to demonstrate the ADC
output.
N = INT ⎟
⎠
⎞
⎜
⎝
⎛
× 5
2
5
127.3
= INT (20.0128) = (20)10= 14H
= (10100)2
Bipolar operation of ADC
A bipolar ADC is one that accepts bipolar input voltage for conversion into appropriate
digital output. The most common bipolar ADCs provide an output called offset binary. This
means that the normal output is shifted by half the scale. So, that all zero corresponds to the
negative maximum input voltage instead of zero
In equation form, the relationship is written as,
N= INT [ n
R
in
V
V
2
2
1
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+ ]
If Vin = -VR/2 , output =0, N=0
If Vin = 0, output is half of 2n
The output will be the maximum count when the input is ⎟
⎠
⎞
⎜
⎝
⎛
− n
R
R
V
V
2
2

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