SENSOR & INSTRUMENTATION

(KOE-044)

Dr Rahat U Khan
Department of Electrical and Electronics Engineering

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 UNIT-04
Data Acquisition Methods
Data Acquisition Methods: Basic block
diagram, Analog and Digital IO, Counters,
Timers, Types of ADC: successive
approximation and sigma-delta, Types of DAC:
Weighted Resistor and R-2R Ladder type, Use of
Data Sockets for Networked Communication

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 DATA ACQUISITION SYSTEMS (DAQ or
DAS)
Data acquisition system (DAS) is a computerized system that collects
data from the real world, converts it into the form of electrical signals
and do required processing on it for storage, and presentation on
computers.
The complete system is controlled and operated by a software
application. This software application is developed by using general-
purpose high-level programming languages like C, C++, java, etc.
These systems are used in industrial and commercial fields. They are
used for collecting, storing and processing of data.
The data acquisition system can be divided into two types:
1) Analog data acquisition system
2) Digital data acquisition system
The analog data acquisition system gives an analog output whereas the
digital data acquisition system gives a digital output.
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Analog DAS is used when wide frequency width is required or when
lower accuracies can be tolerated.
Digital DAS is used when physical quantity being monitored has a
narrow bandwidth (i.e. when the quantity varies slowly). Also, high
accuracy and low per channel cost are required. These are more
complex than analog DAS.
The digital data have more advantages over analog data. Some of
those are:
➢ easy and fast processing,
➢ easy and fast transmission,
➢ easy display,
➢ less storage space is required,
➢ more accurate.
Due to these advantages, mostly the digital data acquisition system is
preferred.

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 Data Acquisition System Block Diagram
A generalized data acquisition system block diagram is shown in
Figure.

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The function of each block is as under:
Transducers: They are converting physical quantities (such as
temperature, pressure, etc.) into electrical quantities, or measuring
electrical quantities directly. They collect data from the physical
world.
Signal Conditioning Unit: The signal produced by the transducers
may or may not be very suitable for our system to work properly. It
may be very weak, very strong or may have some noise.
To convert this signal into the most suitable form, amplification, and
filtration is done respectively by signal conditioning unit. So the signal
conditioning unit converts electrical signals in the most suitable form.
Multiplexer: The multiplexer receives multiple analog inputs and
provides a single output signal according to the requirements.
If a separate channel is used for each quantity, the cost of installation,
maintenance, and periodic replacement becomes high. Therefore, a
single channel is used which is shared by various quantities.
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Analog to Digital (A/D) Converters: The data is converted
into digital form by A/D converters.
After the conversion of data into digital form, it is displayed
with the help of oscilloscopes, numerical displays, panel
meters to monitor the complete system.
Also, the data can be either permanently or temporarily
stored or recorded according to the requirement. The data is
recorded on optical, ultraviolet, stylus or ink recorders for
future use.

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 Objectives of Data Acquisition System

➢ It must collect the necessary data at the correct speed.

➢It must use all the data efficiently to inform the operator
about the state of the system.
➢It must monitor the complete system operation to
maintain on-line optimum and safe operations.
➢It must be able to summarize and store data for the
diagnosis of operation and record purpose.
➢ It must be flexible for future requirements.
➢It must be reliable and not have a downtime of more than
0.1%.
➢It must provide an effective human communication
system.
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The important Factors to Consider When Setting Up a
Data Acquisition System are as follows.
➢ Accuracy and resolution
➢ Number of channels to be monitored
➢ Analog or digital signal
➢ Single channel or multichannel
➢ Sampling rate per channel
➢ Signal conditioning requirements of each channel
➢ Cost

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The various general Configuration of Data
Acquisition System are
Single channel possibilities
➢ Direct conversion
➢ Pre-amplification and direct conversion
➢ Sample and hold, and conversion
➢Pre-amplification, signal conditioning and any of
the above
Multi channel possibilities
➢Multiplexing the outputs of single channel
converters
➢ Multiplexing the output of sample-hold circuits
➢ Multiplexing the inputs of sample-hold circuits
➢ Multiplexing low level data 10
 Applications of Data Acquisition System

The data acquisition system is used in industrial and

scientific fields like aerospace, biomedical and
telemetry industries.

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 Analog to Digital Conversion

There are different types of ADC

➢ Simple Potentiometric & Servo Method
➢ Ramp Type ADC
➢ Integrating Type ADC
➢ Dual-Slope Integrating Type ADC
➢ Successive Approximation Method
➢ Sigma-Delta Type ADC

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 Ramp Type ADC

Input
Voltage

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 Advantages & Disadvantages
of
Ramp Type ADC
Advantages:
➢ Circuit is simple to manufacture
➢ Cost is low
➢Improved Accuracy
Disadvantages:
➢ Slow speed
➢ Many clock pulse may waste in comparison

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Integrating Type ADC

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 Advantages of Integrating Type ADC

Advantages:
➢ It can be used as digital frequency meter
➢There is no effect of variation in R and C
values on frequency fo, because frequency
depends only on input voltage Vin

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Successive Approximation Method

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Successive Approximation type ADC is the most widely
used and popular ADC method. The conversion time is
maintained constant in successive approximation type
ADC, and is proportional to the number of bits in the
digital output, unlike the counter and continuous type A/D
converters. The basic principle of this type of A/D
converter is that the unknown analog input voltage is
approximated against an n-bit digital value by trying one
bit at a time, beginning with the MSB. The principle of
successive approximation process for a 4-bit conversion is
explained here. This type of ADC operates by successively
dividing the voltage range by half, as explained in the
following steps.
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1) The MSB is initially set to 1 with the remaining
three bits set as 000. The digital equivalent voltage
is compared with the unknown analog input voltage.
(2) If the analog input voltage is higher than the
digital equivalent voltage, the MSB is retained as 1
and the second MSB is set to 1. Otherwise, the MSB
is set to 0 and the second MSB is set to 1.
Comparison is made as given in step (1) to decide
whether to retain or reset the second MSB.
The above steps are more accurately illustrated with
the help of an example.

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Let us assume that the 4-bit ADC is used and the analog input voltage
is VA = 11 V. when the conversion starts, the MSB bit is set to 1.
Now VA = 11V > VD = 8V = [1000]2
Since the unknown analog input voltage VA is higher than the
equivalent digital voltage VD, as discussed in step (2), the MSB is
retained as 1 and the next MSB bit is set to 1 as follows
VD = 12V = [1100]2
Now VA = 11V < VD = 12V = [1100]2
Here now, the unknown analog input voltage VA is lower than the
equivalent digital voltage VD. As discussed in step (2), the second
MSB is set to 0 and next MSB set to 1 as
VD = 10V = [1010]2
Now again VA = 11V > VD = 10V = [1010]2
Again as discussed in step (2) VA>VD, hence the third MSB is
retained to 1 and the last bit is set to 1. The new code word is
VD = 11V = [1011]2
Now finally VA = VD , and the conversion stops. 24
It consists of a successive approximation register
(SAR), DAC and comparator. The output of SAR is
given to n-bit DAC. The equivalent analog output
voltage of DAC, VD is applied to the non-inverting
input of the comparator. The second input to the
comparator is the unknown analog input voltage
VA. The output of the comparator is used to activate
the successive approximation logic of SAR.
When the start command is applied, the SAR sets
the MSB to logic 1 and other bits are made logic 0,
so that the trial code becomes 1000.

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 Advantages & Disadvantages of Successive
Approximation Type ADC
Advantages:
➢ Very high speed
➢Conversion time is constant and independent of
the amplitude of the analog input signal VA.
Disadvantages:
➢ Circuit is complex

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Sigma-Delta Type ADC

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The sigma-delta converter is unique in that it samples the
signal in a much higher frequency than the Nyquist
frequency. For that reason it is also called oversampling
converter. It converts the input signal Vin by integrating the
error between a reference signal X5 that can be
either Vref or zero and the input signal. Then, the output of
the integrator X3 is compared with zero. That comparator
result X4 is sampled and sets the reference
signal X5 to Vref or zero in the next cycle. This process is
repeated over and over and the streams of 1s and zeros
coming out of the second comparator average out to the
input value. To convert that bit stream into a binary
code, a decimation filter is used.

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 Advantages & Disadvantages of Sigma
Delta Type ADC
Advantages:
➢ Sigma delta ADC is inexpensive since all circuitry within the
converter is digital
➢ The output of sigma delta ADC is inherently linear
➢ It do not require sample & hold circuit, it is because due to high
sampling rate and low precision
Disadvantages:
➢ It is limited to high resolution and very low frequency
applications.
➢ It is not possible to use sigma delta ADC for multiplexed ac
input signals.
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 Operational Amplifier (Op-amp)
The schematic symbol of an op-amp is shown below.

V1 (Volts) – Non-inverting input voltage.

V2 (Volts) – Inverting input voltage.
V0 (Volts) – Output voltage
V0 = A (V1 - V2 ) 32
 Digital to Analog Conversion
A Digital to Analog Converter (DAC) converts a digital input
signal into an analog output signal. The digital signal is
represented with a binary code, which is a combination of bits
0 and 1.

Types of DACs:
There are two types of DACs
➢ Weighted Resistor DAC
➢ R-2R Ladder DAC 38
Binary Weighted Resistor DAC

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A weighted resistor DAC produces an analog output, which is almost
equal to the digital (binary) input by using binary weighted
resistors in the inverting adder circuit. In short, a binary weighted
resistor DAC is called as weighted resistor DAC.
Recall that the bits of a binary number can have only one of the two
values. i.e., either 0 or 1. Let the 3-bit binary input is b1b2b3. Here,
the bits b1 and b3 denote the Most Significant Bit (MSB) and Least
Significant Bit (LSB) respectively.
The digital switches shown in the above figure will be connected to
ground, when the corresponding input bits are equal to ‘0’. Similarly,
the digital switches shown in the above figure will be connected to the
negative reference voltage, −VR when the corresponding input bits
are equal to ‘1’.
In the above circuit, the non-inverting input terminal of an op-amp is
connected to ground. That means zero volts is applied at the non-
inverting input terminal of op-amp.
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The disadvantages of a binary weighted resistor DAC
are as follows −
➢The difference between the resistance values
corresponding to LSB & MSB will increase as the number
of bits present in the digital input increases.
➢It is difficult to design more accurate resistors as the
number of bits present in the digital input increases.

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R-2R Ladder Type DAC

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The R-2R Ladder DAC overcomes the disadvantages of a binary
weighted resistor DAC. As the name suggests, R-2R Ladder DAC
produces an analog output, which is almost equal to the digital
(binary) input by using a R-2R ladder network in the inverting adder
circuit. Recall that the bits of a binary number can have only one of
the two values. i.e., either 0 or 1. Let the 3-bit binary
input is b1b2b3. Here, the bits b1 and b3 denote the Most Significant
Bit (MSB) and Least Significant Bit (LSB) respectively.
The digital switches shown in the above figure will be connected to
ground, when the corresponding input bits are equal to ‘0’. Similarly,
the digital switches shown in above figure will be connected to the
negative reference voltage, −VR when the corresponding input bits
are equal to ‘1’.
It is difficult to get the generalized output voltage equation of a R-2R
Ladder DAC. But, we can find the analog output voltage values of R-
2R Ladder DAC for individual binary input combinations easily.
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The advantages of a R-2R Ladder DAC are as follows −
➢R-2R Ladder DAC contains only two values of resistor:
R and 2R. So, it is easy to select and design more accurate
resistors.
➢If more number of bits are present in the digital input,
then we have to include required number of R-2R sections
additionally.

Due to the above advantages, R-2R Ladder DAC is

preferable over binary weighted resistor DAC.

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 Example

Let us find the value of analog output voltage of R-2R Ladder DAC for a binary
input, b1b2b3= 100.

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 Digital Counters
Counter is a sequential circuit. A digital circuit which is used for a
counting pulses is known counter. Counter is the widest application of
flip-flops. It is a group of flip-flops and applying a single clock signal
to them.
In simple words, the counters are those, which have the group of
storage elements like flip flops to hold the count.
In counter , the state of the counter is equal to the count held in the
circuit by the flip flops. Counters calculate or note down the number
that how many times an event occurred.
Counters are the crucial hard ware components, and are defined as
“The digital circuit which is used to count the number of pulses”.
Counters have modes. The ‘mod’ of the counter represents the number
of states of the cycles through it, before setting the counter to its
initial state. For example, a binary mod 8 counter has 8 countable
states. They are from 000 to 111. So the mod 8 counter counts from 0
to 7. 57
A binary mod 4 counter has 4 count states, from 000 to 011. So the
mod 4 counter counts from 0 to 3. This means, in general a mod N
counter can contain n number of flip flops, where 2n = N.
It can also be used for Frequency divider, time measurement,
frequency measurement, distance measurement and also for generating
square waveforms.

Need of Counters
Counting means incrementing or decrementing the values of an
operator, with respect to its previous state value. So to perform the
mathematical operation we use no devices other than counters. We
cannot perform this action (counting) with any other logic devices
rather than counters.

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 Types of counters

There are two types of counters available for digital

circuits, they are

➢ Synchronous counters
➢ Asynchronous counters

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 Asynchronous counters
The counters in which the change in transition doesn’t depend
upon the clock signal input is known as “Asynchronous
counters”. In these counters, the first flip flop is connected to the
external clock signal, and the rest are clocked by the state
outputs of the previous flip flop.

Features:
➢Another name for Asynchronous counters is “Ripple
counters”.
➢ These are very simple in design.
➢As its design is simple, they use less number of logic gates to
construct an asynchronous counter.
➢Operation of asynchronous counters is very slow compared to
synchronous counters.
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➢ Different flip-flops are used with a different clock pulse.
➢ All the flip-flops are used in toggle mode.
➢Only one flip-flop is applied with an external clock pulse
and another flip-flop clock is obtained from the output of
the previous flip-flop.
➢The flip-flop applied with external clock pulse acts as
LSB (Least Significant Bit) in the counting sequence.
➢In any counter, last flip-flop acts as MSB (Most
Significant Bit)

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A n-bit ripple counter can count up to 2n states. It is also
known as MOD n counter. It is known as ripple counter
because of the way the clock pulse ripples its way through
the flip-flops.
A counter may be an up counter that counts upwards or can
be a down counter that counts downwards or can do both i.e.
count up as well as count downwards depending on the
input control. The sequence of counting usually gets
repeated after a limit. When counting up, for n-bit counter
the count sequence goes from 000, 001, 010, … 110, 111,
000, 001, … etc. When counting down the count sequence
goes in the opposite manner: 111, 110, … 010, 001, 000,
111, 110, … etc.
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In the circuit shown in above figure, Q1(LSB) will toggle for
every clock pulse because JK flip-flop works in toggle mode
when both J and K are applied 1, 1 or high input. The following
counter will toggle when the previous one changes from 1 to 0.
The 2-bit ripple counter used in the circuit above has four
different states, each one of which represents a count value.
Similarly, a counter having n flip-flops can have a maximum of 2
to the power n states. The number of states that a counter owns is
known as its mod (modulo) number. Hence a 2-bit counter is a
mod-4 counter.
A mod-n counter may also be described as a divide-by-n counter.
This is because the most significant flip-flop (the furthest flip-
flop from the original clock pulse) produces one pulse for every
n pulses at the clock input of the least significant flip-flop (the
one triggers by the clock pulse).
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Timing diagram – Let us assume that the clock is positive
edge triggered so above counter will act as an up counter
because the clock is positive edge triggered and output is
taken from Q.
Counters are used very frequently to divide clock
frequencies and their uses mainly involve in digital clocks
and in multiplexing. The widely known example of the
counter is parallel to serial data conversion logic.

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 Synchronous counters
The counters which use clock signal to change their transition are
called “Synchronous counters”. This means the synchronous counters
depends on their clock input to change state values. All flip flops in
the synchronous counters are triggered by same clock signal.

Features:
➢Their construction is very simple in design. All the flip flops are
interconnected and will be driven by same clock signal.
➢ The state output of the previous flip flop determines the state change
of the present flip flop.
➢As all the flip flops will work synchronously, the synchronous
counters don’t require settling.
➢We require number of logic gates to implement the synchronous
counters.
➢ Their operation is fast.
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 Asynchronous Vs Synchronous Counters

SYNCHRONOUS COUNTERS ASYNCHRONOUS COUNTERS

The propagation delay is very low. Propagation delay is higher than that of
synchronous counters.

Its operational frequency is very high. The maximum frequency of operation is

very low.

These are faster than that of ripple These are slow in operation.
counters.

Large number of logic gates are required Less number of logic gates required.
to design

High cost. Low cost.

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 Applications of counters
Counter found their applications in many digital electronic
devices. Some of their applications are listed below.
➢ Frequency counters
➢ Digital clocks
➢ Analog to digital convertors.
➢With some changes in their design, counters can be used as
frequency divider circuits. The frequency divider circuit is
that which divides the input frequency exactly by ‘2’.
➢In time measurement. That means calculating time in timers
such as electronic devices like ovens and washing machines.
➢We can design digital triangular wave generator by using
counters.
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 555 Timer

The 555 Timer IC got its name from the

three 5KΩ resistors that are used in its voltage divider
network. It very useful in timing related applications.
This IC is useful for generating accurate time delays. It
can be used as alarm generator. It can also be used in
frequency division.

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 Pin Diagram
The 555 Timer IC is an 8 pin mini Dual-Inline Package (DIP). The pin
diagram of a 555 Timer IC is shown in the figure. The significance of
each pin is self-explanatory from the diagram. This 555 Timer IC can
be operated with a DC supply of +5V to +18V. It is mainly useful for
generating non-sinusoidal wave forms like square, ramp, pulse & etc.

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 Functional Diagram
The pictorial representation showing the internal details of a 555 Timer is
known as functional diagram. It contains a voltage divider network, two
comparators, one SR flip-flop, two transistors and an inverter.

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Voltage Divider Network

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Comparator

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Use of Data Socket in Networked Communication

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➢Data Socket is an easy-to-use, high-performance programming
interface that is designed specifically for sharing and publishing live
data in measurement and automation applications.
➢Data Socket for LabVIEW simplifies live data exchange between
different applications on one computer or between computers
connected through a network.
➢Although a variety of different technologies exist today to share
data between applications, including TCP/IP and dynamic data
exchange (DDE), most of these tools are not targeted for live data
transfer to multiple clients. With TCP/IP, you have to convert your
data into an unstructured stream of bytes in the broadcasting
application and then parse the stream of bytes back into its original
format in subscribing applications.
➢Data Socket is a single, unified, end-user application programming
interface (API) for connecting to data from a number of sources –
local files, files on FTP or Web servers, and data items on OPC
Servers. 80
➢A Data Socket application specifies the data location by
using a familiar networking standard.
➢Data Socket Transfer Protocol connects a Data Socket
application to live data by specifying a connection to a Data
Socket Server.
➢Data Socket Server is a stand-alone component, it
simplifies network (TCP/IP) programming by automatically
managing connections to clients and automatically
converting your measurement data to and from the stream of
bytes sent across the network.

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Thank You

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