Biomedical Instrumentation introduction,

1. UEEG002
Medical Instrumentation –
Introduction,Bioamplifiers
D.Poornima,AP(Sr.Gr)/EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore.

2. Introduction
What is Medical Instrumentation??
• Application of biomedical engineering
• Focuses on the devices and mechanics used to measure, evaluate,
and treat biological systems.
• Uses multiple sensors to monitor physiological characteristics of a
human or animal.
• Helps physicians to diagnose the problem and provide treatment,
to measure biological signals and to design a medical instrument.
• Originated as a necessity to constantly monitor vital signs of
Astronauts during NASA's Mercury, Gemini, and Apollo missions

3. Components of Biomedical Instrumentation
System
• Any medical instrument consists of the following functional
basic parts
– Measurand
– Sensor / Transducer
– Signal Conditioner
– Display
– Data Storage and
Data Transmission

4. • Measurand
– The measurand is the physical quantity,property or condition that instrumentation
system measures. Human body acts as the source for measurand, and it generates
bio-signals.
Example: Blood pressure (Internal)
Electrocardiograms (on the body surface)
Blood or Biopsy (Emanate from the body)
• Sensor / Transducer
– Converts one form of energy to another form usually electrical energy.
– The transducer produces a usable output depending on the measurand.
– The sensor is used to sense the signal from the source. It is used to interface
the signal with the human
Example: Piezoelectric signal which converts mechanical vibrations into the
electrical signal

5. • Signal Conditioner
– Used to convert the output from the transducer into an electrical
value.
– The instrument system sends this quantity to the display or recording
system.
– signal conditioning process includes amplification, filtering, analogue to
digital and Digital to analogue conversions.
– Signal conditioning improves the sensitivity of instruments.
• Display
– Provides a visual representation of the measured parameter or
quantity.
Example: Chart recorder, Cathode Ray oscilloscope (CRO).
– Sometimes alarms are used to hear the audio signals.
Example: Signals generated in Doppler Ultrasound Scanner used for
Fetal Monitoring.

6. • Data Storage and Data Transmission
– Data storage is used to store the data and can be used for future reference.
– Recent days Electronic Health records are utilized in hospitals.
– Data transmission is used in Telemetric systems, where data can be transmitted from
one location to another remotely.

7. Bioelectric Potential
• Human body has many subsystems inside it like
– Cardiovascular system
– Respiratory system
– Nervous system
– Digestive system
• The systems in the human body generate their own monitoring signals when
they carry out their functions.
• These signals provide useful information about their function.
• These signals are bioelectric potentials associated with nerve conduction,
brain activity, heartbeat, muscle activity and so on.
• Bioelectric potentials are actually ionic voltages produced as a result of
electro chemical activity of certain cell.
• Transducers are used to convert these ionic potentials in to electrical signals

8. Cell Structure

9. • Certain types of cells within the body, such as nerve and muscle cells are
encased in a semi permeable membrane.
• This membrane permits some substances to pass through while others
are kept out.
• Surrounding the cells of the body are the body fluids.
• These fluids are conductive solutions containing charged atoms know as
ions.
• The principle ions are sodium (Na+) Potassium (K+) and chloride (Cl-).
• The membrane of excitable cells permits entry of
Potassium (K+) and chloride(C-) ions but blocks
the entry of sodium (Na+) ions.

10. Resting Potential
• So potential inside the cell is more negative than outside cell.
• This membrane potential is called Resting potentials.
• This potential is measured from inside the cell with respect to body fluids.
• So resting potential of a cell is negative.
• This resting potential ranging from -60mv to -100 mv.
• Cell in the resting state is called polarized cell.

11. Action Potential [Depolarization]
• When a section of a cell membrane is excited by the flow of
ionic current or by some form of externally applied energy,
the membrane allows some Na+ and try to reach some
balance of potential inside and outside.
• Same time some K+ goes outside but not rapidly like
sodium.
• As a result, the cell has slightly Positive potential on the
inside Due to the imbalance of the Potassium ions.
• This potential is known as “action potential” and is
approximately +20 mV

12. • A cell that has been excited and that displays an action potential is said to
be depolarized
• The process from resting to action potential is called depolarization.
• Action potential is always positive inside.

13. Reploarization
• The process of the cell coming to
depolarised state from polarised state

14. Characteristics of Resting and Action Potential

15. Bio potential Amplifiers
– Very important part of modern medical instrumentation
– To amplify biopotentials which are generated in the body at
low levels with high source impedance
– are required to increase signal strength while maintaining
fidelity
• Functions of Biopotential Amplifiers
– To take a weak bio-potential and increase its amplitude so
that it can be processed, recorded or displayed
– To amplify voltage, power and current.
– Sometimes used to isolate the load from the source current
gain only

16. • Characteristics of Biopotential Amplifiers
– High input impedance
– Low output impedance
– The biopotential amplifier must be sensitive to important
frequency components of the biosignal
– Biopotential amplifiers have a gain of 1000 or greater
– Most biopotential amplifiers are differential
– High common mode rejection ratio.

17. Types of Bio Amplifiers
1. Differential Amplifier
2. Operational Amplifier
3. Instrumentation Amplifier
4. Isolation Amplifier

18. Differential Amplifier
• Type of electronic amplifier that amplifies the difference between two
input voltages but suppresses any voltage common to the two inputs.
• A differential amplifier is an analog circuit with two inputs (V1 and V2)
and one output (V0) in which the output is ideally proportional to the
difference between the two voltages.
• The formula for a simple differential amplifier can be expressed:
• V0 is the output voltage
• V1 and V2 are the input voltages
• Ad is the gain of the amplifier (i.e. the differential amplifier gain)

19. • When V1 = V2, V0 is equal to zero, and hence the output voltage is
suppressed.
• But any difference between inputs V1 and V2 is multiplied
by the differential amplifier gain Ad.
• It is also known as a difference amplifier
– the difference between the input
voltages is amplified.

20. Operational Amplifier
• Op amp is basically a multistage amplifier in which several amplifier stages are
interconnected to each other in a very complicated manner.
• Its internal circuit consists of many transistors, FETs and resistors. All this occupies
a very little space.
• It is packed in a small package and is available in the Integrated Circuit (IC)form.
• The term Op Amp is used to denote
an amplifier which can be configured
to perform various operations like
amplification, subtraction,
differentiation, addition, integration etc

21. • Applying two signals, one at the inverting and another at the non-
inverting terminal, op-amp will amplify the difference between
them
• The difference between two input signals is called as the
differential input voltage.
• The equation below gives the output of an operational amplifier.
• VOUT is the voltage at the output terminal of the op-amp.
• AOL is the closed-loop gain for the given op-amp and is constant (ideally)
• The feedback is negative if the feedback path feeds the part of
the signal from the output terminal back to the inverting (-)
terminal.
• We use negative feedback to the op-amps used as amplifiers

22. Chopper Amplifier
• This circuit uses a chopping device which converts slowly varying dc
to an alternating form with amplitude proportional to the input
direct current and phase depends on the polarity of original signal.
• The ac voltage is then amplified by an ac amplifier whose output is
then rectified back to get an amplified direct current.
• Figure shows single ended chopper stabilizer amplifier.
• Aim of this circuit is to avoid the dc offset voltage present in the
output signal.
• For that purpose first convert the dc signal into ac using a chopper

24. Components
• Low Pass Filter
– The low frequency components derived from the input signal by passing it through low
pass filter R2C2 and R2
• Chopper
– The output of LPF is then chopped using a transistor switch w.r.t a carrier signal from
the oscillator.
• Demodulator
The original signal recovered in demodulator which is again applied to second stage of
amplification.
• Low Pass Filter
– Before given to the amplifier low frequency components again derived using LPF
• Second Stage of Amplification (A2)
– At the input of A2 an HPF C1R1 is used to derive the high frequency signals.
– This is to reduce the dc offset and drift of second amplifier A2.

25. Instrumentation Amplifier
• Commonly used Bio-potential Amplifier is Differential amplifier. But it has
some limitations.
• These limitations overcome with the availability of improved version of
differential amplifier called Instrumentation Amplifier
• High gain and the high input impedance are attained with an instrumentation
amplifier.

26. • Usually, a 3-amplifier and 7 resistor setup forms the
instrumentation amplifier circuit.
• The output from the transducer is given as input to the
instrumentation amplifier.
• Before the signal goes to the next stage, a special amplifier is
required with high CMRR, high input impedance and to avoid
loading effects.
• Two buffer amplifiers A1 and A2 connected to a differential
• amplifier A3.
• Op-amp A3 with four equal resistors R form a differential
• amplifier with gain 1.
• Rg = used to set the gain using the formula

28. • Advantage of Chopper Amplifier
 Insensitivity to component changes due to ageing, temperature change, power
supply variation, or other environmental factors.
 Small offset voltage
 Used to amplify small dc signals of few microvolts.

29. Isolation Amplifier
• Used for protection against leakage currents
• Isolation between input and output
• Isolation between different supply voltages
and grounds
• Three methods used in the design
– Transformer Isolation
– Optical Isolation
– Capacitive Isolation

30. • Transformer Isolation
– Uses Frequency modulated or a pulse width modulated carrier signal
– Uses internal DC-DC converter
comprising of a 20kHz oscillator,
transformer, rectifier, and filter
to supply isolated

31. • Optical Isolation
– Can also be achieved by optical means
– Patient is not connected electrically to hospital line or ground
– Separate battery supplies power to the patient circuit
– Patient bio signal is converted into a light by a light source
– On output side, photo transistor
converts light signal again to
electrical signal

32. • Capacitive Isolation
– Uses digital encoding of the input voltage and frequency modulation to
send the signal across a differential capacitive barrier
– Separate power supply needed
on both sides of the barrier