Digital Instrumentation

1. Chapter 6:
Digital Instrumentation
Compiled by:
Bishal Rimal
Lectrurer, KEC
Dhapakhel, Lalitpur Nepal

2. Syllabus (5hours)
• 6.1 Sample data system, sample and hold circuit
• 6.2 Components of data acquisition system
• 6.3 Interfacing to the computer

3. Revision
• Signals are those function that conveys information about the behavior or attribute
of some phenomena.
• Analog and Digital are the different forms of signals. Signals are used to carry
information from one device to another. Analog signal is a continuous wave that
keeps on changing over a time period. Digital signal is discrete in nature. The
fundamental difference between analog and digital signal is that analog signal is
represented by the sine waves whereas, the digital signal is represented by square
waves.
• Instrumentation system can be either analog or digital instrumentation system

4. Data Sampling
• Sampling is the reduction of continuous time signal to discrete time signal.
• Sample is the value or set of value at a point in a time or space. To process
the analog signal digitally, it has to be digitalized first or sliced first.
• Sampling is simply multiplication of analog signal by the train of pulses of
unit magnitude
• For efficient use of data processing and data transmission.

5. a: Analog signal to be sampled
b: Unit signal pulse
c: Output of multiplication of signal
a and signal b
d : Holding of signal until next pulse
Fig: Signal Sampling Pulse
• Time period between successive sample is
called sampling interval or sampling
period.
• Results out a series of narrow pulse.
• Magnitude of narrow pulse represents
magnitude of analog signal at sampling
instant.

6. Sampling Theorem (Nyquist Criteria)
• If Sampling interval is less, more sampled signal are obtained for a cycle.
• More sampled signal means more representation of analog signal.
• If sampling interval is more, there might be loss of property of analog signal.
• “A continuous time signal can be represented in its samples and can be recovered back
when sampling frequency fs is greater than or equal to the twice the highest frequency
component of message signal fm.”
i.e
• The sampling rate is called Nyquist rate and condition is called nyquist criteria.
• In real application sampling is done at 2.5 to 5 times highest frequency component of
analog signal

7. Aliasing
• Failure to meet that Nyquist criteria results in a sample which represents some other
analog signal than the original input signal, this effect is termed as “aliasing” and the
resultant signal is called Alias.
• Thus, Aliasing occurs when a system is measured at an insufficient sampling rate.
• Let analog signal m(t)= 3cos50pt+2sin300pt-5cos100pt
• Here for max w=300p i.e fm=150
• As per nyquist criteria, fs≥2fm so, fs≥2*150=300Hz
• If fs is less than 300 Hz aliasing occurs.

8. Aliasing
a
b
c
a: Sampled twice per period
(Very poor interpretation of
signal, doesnt give true
intelligence but correctly
represent the period of signal)
b: Sampled 3/2 times per
period
c: less than one period per
cycle

9. Anti Aliasing
• The sampling rate for an analog signal must be at least two times as high as the
highest frequency in the analog signal in order to avoid aliasing.
• Conversely, for a fixed sampling rate, the highest frequency in the analog signal can
be no higher than a half of the sampling rate. Any part of the signal or noise that is
higher than a half of the sampling rate will cause aliasing.
• In order to avoid this problem, the analog signal is usually filtered by a low pass
filter prior to being sampled, and this filter is called an anti-aliasing filter.
• Thus
• Use of Low pass filter before sampling for a fixed sampling rate devices
• Make sampling frequency greater than or equals to maximum analog input frequency.

10. Maximum allowable rate of change of input and maximum input
frequency applied to ADC
Suppose a sinusoidal input
Vi=VmSinwt is applied to n-bit ADC
Suppose the full range of the n-bit ADC
is Vm and its conversion time is Tc,

11. Sample and Hold Circuit
• The Sample and Hold circuit is an electronic
circuit which creates the samples of voltage
given to it as input, and after that, it holds these
samples for the definite time.
• The time during which sample and hold circuit
generates the sample of the input signal is
called sampling time (Ts).
• Similarly, the time duration of the circuit during
which it holds the sampled value is
called holding time (Th).

12. S and H circuit cntd…
M1 Closed: Sample mode
M1 open: Hold mode
Analog input
• Figure shows the simplest form of S&H
circuit.
• When switch is closed: Vc follows Vin i.e
Vin=Vout. (Sample mode)
• When switch is open: Vc is hold and
Vout=Vc (Hold mode)
• The fast switching is achieved by MOSFET,
Thus practical S&H circuit consist of
MOSFET followed by voltage follower op-
amp circuit.

13. Voltage follower
Major Components
• N-channel Enhancement type MOSFET: for fast
Switching
• Capacitor: to store and hold the electric charge
• Voltage follower with high precision operational
amplifier.
Ts Th
Note: Blue one is the sampled and hold data

14. Working of S&H circuit
• The N-channel Enhancement MOSFET will be used a switching element. The input voltage is applied through
its drain terminal and control voltage will be applied through its gate terminal. When the positive pulse of the
control voltage is applied, the MOSFET will be switched to ON state. And it acts as a closed switch. On the
contrary, when the control voltage is zero then the MOSFET will be switched to OFF state and acts as the open
switch.
• When the MOSFET acts as a closed switch, then the analogue signal applied to it through the drain terminal will
be fed to the capacitor. The capacitor will then charge to its peak value. When the MOSFET switch is opened,
then the capacitor stops charging. Due to the high impedance operational amplifier connected at the end of the
circuit, the capacitor will experience high impedance due to this it cannot get discharged.
• This leads to the holding of the charge by the capacitor for the definite amount of time. This time can be
referred as holding period. And the time in which samples of the input voltage is generated is called sampling
period.
• The output processed by operational amplifier during the holding period. Therefore, holding period holds
significance for OP-AMPS.

15. Performance Parameters
• Aperture time(Tp): The aperture time can be defined as
the time required by the capacitor to change its state
from sampling to holding. Due to the propagation delay
of switches, even after the hold command is given the
capacitor still keeps on charging for the short duration
of time. This is nothing but aperture time.
• Acquisition Time (Ta): Suppose S&H circuit is in hold
mode, and a positive voltage is applied to gate. Ideally
circuit must go in sample mode instantaneously. But in
practice, there is certain delay in time known as
Acquisition time.
• Voltage Droop: The voltage droop is the voltage drop
down in the capacitor due to leakage of charge by a
capacitor.

16. Application of Sample and Hold Circuit
• Data Distribution System
• Sampling Oscilloscopes
• Data Conversion System
• Digital Voltmeters
• Analog Signal Processing
• Signal Constructional Filters

17. Assignment 6.1
• Define: analog signal, Digital Signal and Sampling. [4]
• State and Explain Sampling theorem. What is Aliasing and how to avoid it.
[8]
• What is Sample and hold circuit? Discuss its operation with its basic circuit
and waveform. What is its purpose of using S & H circuit in ADC? [8]
• Explain Aperture time and acquisition time with suitable waveform. [4]

18. Data Acquisition System
• Somewhere instrumentation system should be able to transmit two or more signals at
the time over a single link. The need for this comes from the necessity to measure and
record large quantities of data in research, development and production.
• 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.
• Examples of data acquisition systems include such applications as weather monitoring,
recording a seismograph, pressure, temperature and wind strength and direction. This
information is fed to computers, which then predict natural events like rain and
calamities like destructive winds.
• An example of a DAS in the medical field is a patient monitoring system that tracks
signals like an ECG (Electro-cardiogram) or EEG (Electro-encephalogram).

19. Objective of DAS
• It must acquire the necessary data, at correct speed and at the correct time.
• It must use of all data efficiently to inform the operator about the state of the plant.
• It must monitor the complete plant operation to maintain on-line optimum and safe operations.
• It must provide an effective human communication system and be able to identify problem areas,
thereby minimizing unit availability and maximizing unit through point at minimum cost.
• It must be able to collect, summarize and store data for diagnosis of operation and record purpose.
• It must be able to compute unit performance indices using on-line, real-time data.
• It must be flexible and capable of being expanded for future require
• It must be reliable, and not have a down time greater than 0.1%.
Soucre:http://www.eeeguide.com/data-acquisition-system/

21. Analog DAS
(1) transducers to convert physical parameters to
electrical signals,
(2) signal conditioners for amplifying, refining or
selecting certain portions of these signals,
(3) a multiplexer for scanning different signal sources,
(4) visual-display devices for continuous
monitoring of the signal,
(5) graphic recorders for obtaining permanent
records of input data and
(6) magnetic recorders for preserving and
reproducing data.

22. Digital DAS
• In general analog DASs are used for measurement systems with wide bandwidth. But
the accuracy is less. So digital DASs which have high accuracy, low per channel cost and
narrow bandwidth (slowly varying signal) are designed. The function of the digital data
acquisition system include handling analog signals, making the measurement, converting
and handling digital data, internal programming and control.

23. • Here, the transducer translates physical parameters to electrical signals acceptable by the acquisition
system. The physical parameters include temperature, pressure, acceleration, weight, displacement,
velocity etc. Electrical quantities such as voltage, stance, and frequency may be measured directly.
• The signal conditioner includes the supporting circuitry for the transducer. This circuit may provide
excitation power, balancing circuits and calibration elements and an example of this is a strain-gauge
bridge lance and power supply unit
• The scanner or multiplexer accepts multiple analog inputs and sequentially connects them to one
measuring instrument.
• The signal converter translates the analog signal to a form acceptable by the analog to digital converter
like an amp1ifier used for amplifying low-level voltages generated by thermocoup1es or strain gauges.
• The analog to digital converter (ADC) converts the analog voltage to its equivalent digital form.
• The output of the ADC may displayed visually and is also available as voltage outputs indiscrete steps
for further processing or recording on a digital recorder.
• The auxiliary section contains instruments for system programming and digital data processing such as
linearizing and limit comparison. These functions may be performed by individual instruments or by a
digital computer.
• The digital recorder records digital information on punched cards, perforated paper tape, magnetic
tape, typewritten pages or a combination of these systems. Digital recorder may be preceded by a
coupling unit that translates the digital information to the proper form for entry into particular digital
recorder selected.

24. Conclusion on DAS
• Data acquisition systems typically convert analog Physical condition into
digital values for easy processing.
• DAS is advantageous as we can store a lot of physical condition data in
digitized form
• DAS helps in easy processing of data as well as easy comparison can be
done.
• Today DAS is used in almost every field, industry and companies.

25. Interface
• What is interface?
• How do you connect USB to computer, earphone to mobile phones?
• Man-Machine Interface  Accelerator to ICE
• Interface {Plugs, Cables, Software} defined by International Standards
• A boundary across which two independent systems meet and act on or communicate with each
other. In computer technology, there are several types of interfaces.
• user interface - the keyboard, mouse, menus of a computer system. The user interface allows the
user to communicate with the operating system.
• software interface - the languages and codes that the applications use to communicate with each
other and with the hardware.
• hardware interface - the wires, plugs and sockets that hardware devices use to communicate with
each other.
• To connect with or interact with by means of an interface.

26. • Examples:
• Intel 8212- I/O controller of 8 bits
• Intel 8155-Programmable device
• Intel 8251C- Microcontroller
• RS 232- Bus for serial communication
• Intel 8279-Keyboard display interface device
Interfacing to computers

29. Thank You!!