1. Unit 4
Data Acquisition and
conversion
By Tejas Prajapati
Reference book:
1. Electronics Instrumentation by H. S. Kalsi
2. Electronics Device & circuit –II by Jigar H. Shah.
3. Communication Systems by Sanjay Sharma
2. Data acquisition System
• A typical data acquisition system consists of individual sensors with
the necessary signal conditioning, data conversion, data processing,
multiplexing, data handling and associated transmission, storage and
display systems.
• Analog data generally acquire and convert into digital form for the
purpose of processing, transmission, display and storage.
• Data acquisition generally relates to the process of collecting the input
data in digital form as rapidly, accurately and economically as
necessary
3. Block Diagram of General Data acquisition
System (DAS)
Transducer 1
Transducer 2
Transducer 3
Transducer 4
Signal
conditioner 1
Signal
conditioner 2
Signal
conditioner 3
Signal
conditioner 4
Multiplexer A/D Converter
Printer
Digital
Display
Magnetic
Tape
Transmissio
n
Computer
processing
Analog
Data
-Recorder
-Display
-Meter
4. • Transducer :Transducer which converts the any energy in to electrical
signal.
• Signal conditioner : To match the input requirement with output of
sensor or transducer, some form of scaling and offsetting is necessary,
and this achieve by use of Signal conditioner.
- E.g differentiator, integrator, rectifier, phase detection, divider etc.
• Multiplexer: For converting analog information from more than one
source, either additional transducers or multiplexers employed. To
increase the speed with which information accurately converted,
sample-hold circuit are used.
• A/D Converter: It is used for DAS application usually designed to
receive external command to convert and hold.
5. Data Loggers
• The basic function of data loggers is to automatically make a record od
readings of instruments located at different parts of plant.
• Data loggers measure and record data effortlessly as quickly, as often
and as accuracy desired.
6. Data Loggers block diagram
Input Scanner
Signal
amplifier or
conditioner
A/D Recorder
Real time
clock
Programmer
I
n
p
u
t
Convert
Record
Start
7. • The i/p scanner is an automatic sequence which selects each signal in turn.
• Low level signal, if any are multiplied to bring them up to level of 5 V.
• If the signal are not linearly proportional to measure parameter, these
signal are linearized by the signal conditioner.
• The analog signal are then converted to digital signal for driving the
recording equipment. (Printer or punch card tape)
• Programmer is used to control the sequence operation of various items of
the logger.
• It tells scanner when to step to new channel, receivers information from
the scanner, converter and recorder.
• The clock commands the programmer to sequence one set of
measurement at intervals selected by the user.
8. Sampling Theorem
•Statement : It states that, “ A continuous time
signal may be completely represented in its
samples and recover back id the sampling
frequency is fs≥2fm. Here fs is sampling frequency
and fm is the maximum frequency present in the
signal.
9. Proof
• Let consider continuous time signal x(t)
whose spectrum band limit to fm Hz. Means
that the signal has no limit beyond the fm.
• So X(ω)=0 for | ω|> ωm
• Where ωm=2πfm
• Fig a. and fig. b
x(t)
t0
Fig. a : Continuous signal
X(ω)
ω
f
0
0
ωm-ωm
fm fm
Fig. b : Spectrum of continuous time signal
10. • Sampling of x(t) at rate of fs may be
achieve by multiplying x(t) and impulse
train δTs(t). (fig. d).
• Ts= Sampling interval
• The resulting or sampling signal may be
written as
g(t)= x(t)*δTs(t)_____(1)
• Impulse train δTs(t) may be express as
trigonometric Fourier series
δTs(t)=1/Ts[1+2cos ωst +2cos 2ωst+ 2cos
3ωst+ …] ____(2)
Where ωs = 2πfs
• Put eq. (2) in to (1).
δTs(t)
t…………. -2Ts -Ts 0 Ts 2Ts …………..
Multiplier
x(t)
δTs(t)
g(t)
Fig. c: Impulse train as sampling
function
Fig. d: Multiplication of x(t) and δTs(t)
11. • So, g(t) = 1/Ts[x(t)+2x(t)cos ωst +2x(t)cos 2ωst+ 2x(t)cos 3ωst+ …]____(3)
• Fourier transform of g(t) is G(ω),
Fourier transform of x(t) is X(ω)
Fourier transform of 2x(t)cos ωst is [X(ω-ωs)+X(ω+ωs)]
Fourier transform of 2x(t)cos 2ωst is [X(ω-2ωs)+X(ω+2ωs)]
So on…
• Therefore Fourier transform of g(t) can be written as
G(ω)=1/Ts[X(ω)+X(ω-ωs)+X(ω+ωs)+X(ω-2ωs)+X(ω+2ωs)+…] ____(4)
Or
G(ω)=1/Ts Σ X(ω-nωs) ____(5)
n=-∞
∞
12. • From eq. (4) and (5), it is clear that the
spectrum G(ω) consists of X(ω) reaping
periodically with period ωs . Shown in fig f.
• Now if have to reconstruct x(t) from g(t), we
must be able to recover X(ω) from G(ω). This is
possible if there is no overlap between
successive cycle G(ω). Fig. f shows that this
requires
fs> 2fm ____(6)
But the sampling interval Ts=1/ fs,
Hence Ts<1/ 2fm ___(7)
• Therefore, as long as sampling frequency fs is
greater than twice the maximum frequency fm .
g(t)
t0
Fig. e: Sampled signal
ω
f
G(ω)
-2ωs –ωs 0 ωs 2ωs
-2fs –fs –fm 0 fm fs 2fs
Fig. f: Spectrum of
sampled signal
13. • X(t) can be recovered from its sample signal g(t) by passing through
an ideal low pass filter of band limit or cutoff frequency fm Hz.
• This is proves the sampling theorem.
14. Sample and hold circuit
Basic block diagram of sample and hold circuit.
• The sample and hold (S/H) circuit is used to
sample an analog input for very short period
and hold its sampled value until the input is
sampled again.
• The S/H circuit has three terminal : Input,
Output and logic S/H command.
• Logic S/H command called S/H control
voltage.
• Logic S/H command is generally very small
width pulse.
• The S/H circuit is the basic block of the
analog to digital converter (ADC), digital
communication and digital instruments.
15.
16. • The control voltage is to connected to JFET/MOSFET.
• JFET/MOSFET works like a switch.
• Capacitor CH is hold capacitor and serves as storage element.
• The analog input is applied to the drain of JFET/MOSFET and control
voltage applied to the gate of JFET/MOSFET.
• When positive valued control voltage is applied to JFET/MOSFET
gate, the JFET/MOSFET conduct and act like closed switch.
• The capacitor is charge through input voltage and due to unity gain
non-inverting op-amp circuit is available at the output.
• When control voltage is removed, JFET/MOSFET is off and act as
open switch.
• The capacitor is discharge through op-amp only, but due to its high
input impdence, the capacitor holds voltage.
17. • The circuit modifies the capacitor voltage and hence output when
control signal is applied.
• Usually the control signal frequency must be twice the input signal
frequency to avoid the distortion and reproduce the analog signal back
from sampled signal. This is called Nyquist sampling criteria (or
sampling theorem).
