Subject: Sensor & Instrumentation Subject Code: BOE-405

Important Q&A-Unit-4

1. Describe the Operation of Counters and Timers in Data Acquisition

System. (2023-24) (7 marks)

A timer is a specialized type of clock which is used to measure time intervals. A timer that
counts from zero upwards for measuring time elapsed is often called a stopwatch. It is a
device that counts down from a specified time interval and used to generate a time delay, for
example, an hourglass is a timer.

A counter is a device that stores (and sometimes displays) the number of times a particular
event or process occurred, with respect to a clock signal. It is used to count the events
happening outside the microcontroller. In electronics, counters can be implemented quite
easily using register-type circuits such as a flip-flop.

Counter and timer functions can be programmed to start and stop external hardware (or
software) when certain logical conditions in the data acquisition system have been met, such
as turning on a valve after a specific number of pulses are counted in a particular time period.
It also enables to select if we want to measure position with an encoder, measure a frequency,
output a pulse train.

2. Discuss basic Block Diagram of Data Acquisition System and objectives

of DAQ. What are the 3 R’s for DAQ? (2023-24) (7 marks)

Fig: Block Diagram of DAS/DAQ

The block diagram of a general Data Acquisition System (DAS) is shown in the figure. It
consists of the following elements.
1. Transducer-A transducer is used to convert the physical parameters corning from the field
into electrical signals or it is used to measure directly the electrical quantities such as
resistance,
voltage,frequency,etc.

2. Signal Conditioner-Usually the output signals of the transducer will be of very low level
(weak) signals which cannot be used for further processing. In order to make the signals
strong enough to drive the other elements signal conditioners such as amplifiers, modifiers,
filters etc., areused.

3. Multiplexer-The function of the multiplexer is to accept multiple analog inputs (after

signal
conditioning) and provide a single output sequentially according to the requirements.

4. A/D Converter-The analog-to-digital (A/D) converter is generally used to convert the

analog data into digital form. The digital data is used for the purpose of easy processing,
transmission, digital display and storage. Processing involves various operations on data such
ascomparison, mathematical manipulations, data is collected, converted into useful form and
utilizedforvarious purposes like for control operation and display etc. The transmission of
data in digital form is possible over short distances as well as long distances of and has
advantages over transmission in analog form. The data can be stored permanently or
temporarily and can be displayed on a CRT or digital panel.

5. Recorders and Display Devices-In display devices the data is displayed in a suitable form
in order to monitor the input signals. Examples of display devices are oscilloscopes,
numerical displays, panel meters, etc. In order to have either a temporary or permanent record
of the useful data recorders are used. The analog data can be recorded either graphically or on
a magnetic tape. Optical recorders, ultraviolet recorders, styles-and-ink recorders are some of
its examples. The digital data can be recorded through digital recorders. The digital data is
first converted into a suitable form for recording by means of a coupling unit and then
recorded on a magnetic tape, punched cards or a perforated paper tape

The 3R’s of DAQ are Resolution, Range and Rate.

3. Explain Operation of an R-2R ladder DAC. (2023-24) (7 marks)

As its name implies, the “ladder” description comes from the ladder-like configuration of the
resistors used within the network. A R-2R resistive ladder network provides a simple means
of converting digital voltage signals into an equivalent analogue output.

Input voltages are applied to the ladder network at various points along its length and the
more input points the better the resolution of the R-2R ladder. The output signal as a result of
all these input voltage points is taken from the end of the ladder which is used to drive the
inverting input of an operational amplifier.

Then a R-2R resistive ladder network is nothing more than long strings of parallel and series
connected resistors acting as interconnected voltage dividers along its length, and whose
output voltage depends soley on the interaction of the input voltages with each other.
Consider the basic 4-bit R-2R ladder network (4-bits because it has four input points) below.
4. Discuss Successive approximation types (SAR) ADC with one example.
(2023-24) (7 marks)
5. Explain working of binary weighted resistor type digital to analog
converter. (2023-24) (7 marks)

The Digital to Analogue Converter, or DAC’s as they are more commonly known, are the
opposite of the Analogue to Digital Converter. DAC’s convert binary or non-binary numbers
and codes into analogue ones with its output voltage (or current) being proportional to the
value of its digital input number.

For example, we may have a 4-bit digital logic circuit that ranges from 0000 to 11112, (0 to
F16) which a DAC converts to a voltage output ranging from 0 to 10V.

Converting an “n”-bit digital input code into an equivalent analogue output voltage between 0
and some VMAX value can be done in a number of ways and one method is use of weighted
resistors and operational amplifier.
4-bit Binary Weighted Digital-to-Analogue Converter

For a 4-bit binary number there are 24 = 16 possible combinations or A, B, C, and D ranging
from 00002 to 11112 which corresponds to decimal 0 to 15 respectively. If we make the
weight of each input bit double with respect to the other, we end up with an 8-4-2-1 binary
code ratio corresponding to 23, 22, 21 and 20.

So if we set the “D” input resistance at 1kΩ, the “C” input resistance at 2kΩ (that is the
double of D), the “B” input resistance at 4kΩ (double C), and the “A” input resistance at 8kΩ
(double B), with the feedback resistance RF set again at 1kΩ, then theoutput voltage/gain of
the 4-bit binary weighted digital-to-analogue converter would be:

with the 4-bit binary code of 0001 producing an output voltage of -0.625 volts.

6. Explain in detail Counter type A/D converter; write the applications of

analog to digital converter (ADC). (2023-24) (7 marks)

The counter-type Analog-to-Digital Converter (ADC) is also known as the digital ramp ADC.
It is because the output of the counter is fed to a Digital-to-Analog Converter (DAC), and
while the counter increments its count, the output of the DAC increases in ramp fashion or
staircase fashion.

The counter-type ADC uses a counter for conversion from analog to digital.
Counter-Type ADC Operation

During the start of the conversion, the output of DAC is zero. So, whatever input voltage Vin
is applied at the positive terminal of the comparator, the output of the comparator is high.
Since it is high, the AND gate is enabled and it allows the clock pulse to pass. The counter
then starts counting the clock pulses. The output of the counter is fed to the DAC, which
computes the decimal equivalent of its binary input. Now, the output of the DAC VDAC
increases in a staircase fashion and it is continuously compared with the input Vin. As long as
Vin > VDAC, the counter keeps counting.

The moment Vin < VDAC, the comparator output is low, and AND gate is disabled, therefore
blocking the clock pulses. Also, the control block notices this transition and puts a low signal
in the clear pin of the counter, thereby resetting it. Simultaneously, the last output of the
counter is latched and this is the digital binary output of the given input voltage.

So, the basic principle of operation of the counter-type ADC is to keep counting the number
of clock pulses till the input is greater than the DAC output and the moment DAC output is
greater than the input, the counter is reset and the last count is latched and given as output.

Counter-type ADC Conversion Time

Conversion time for any ADC is defined as the time taken by it to convert a given analog
input to a digital binary output. It is analogous to the propagation delay we study in digital
logic gates.

If the counter has to count from the zeroth state (that is all 0s) and counts up to N, then the
conversion time is (N-1)Tc, where Tc is the time period of the clock pulse. Maximum
conversion time occurs when the input voltage is equal to the full-scale output range of the
DAC. The DAC gives a full-scale output when all bits are one. To reach all ones from all
zeroes, the counter takes 2n-1 clock pulses, where n is the number of bits. Hence, maximum
conversion time = (2n-1)Tc.

Advantages

The counter-type ADC is simple and easy to use. It is very accurate and the precision can be
increased by increasing the number of bits. While there are many complicated ADCs in the
market, counter-type ADC has a fair balance of accurate output and cheap hardware costs.

Disadvantages

The major drawback of this ADC is that every time a new conversion starts, the counter is
reset and it has to start counting right from all zeroes. Hence, the conversion time is
significant. It cannot sustain high-frequency input, as it will not be able to convert the analog
signal to digital binary in real-time. The worst case occurs when the input voltage is equal to
the full-scale output range of the DAC.
7. Compare successive approximation and sigma delta ADCs. (2023-24) (7
marks)
8. Explain difference between counter and timer. (2023-24) (7 marks)

A timer is a specialized type of clock which is used to measure time intervals. A timer that
counts from zero upwards for measuring time elapsed is often called a stopwatch. It is a
device that counts down from a specified time interval and used to generate a time delay, for
example, an hourglass is a timer.

A counter is a device that stores (and sometimes displays) the number of times a particular
event or process occurred, with respect to a clock signal. It is used to count the events
happening outside the microcontroller. In electronics, counters can be implemented quite
easily using register-type circuits such as a flip-flop.

Difference between a Timer and a Counter

The points that differentiate a timer from a counter are as follows –

Timer Counter
The register incremented for every machine The register is incremented considering 1 to
cycle. 0 transition at its corresponding to an
external input pin (T0, T1).
Maximum count rate is 1/12 of the oscillator Maximum count rate is 1/24 of the oscillator
frequency. frequency.
 A timer uses the frequency of the internal A counter uses an external signal to count
clock, and generates delay. pulses.

9. What is use of Data Sockets for Networked Communication? (2023-24)

(7 marks)
10. Explain the working principle of Flash type A/D Converter. (2023-24)
(7 marks)
 Q11. A 4- bit DAC has Vref = 12 V. For an input of 1100, find
the equivalent analog output voltage given by DAC.

Q12. A 6 bit DAC has a step size of 50 Mv. Determine the FSV and %
resolution.
Q13. Determine the resolution of a a) 16 bit DAC b) 12 bit DAC in terms of
%.

Q14. An 8 bit DAC produces output voltage of 0.05 V for a digital input of
00000001. Find the FSV. What is the resolution? What is output voltage for
an input of 00101010 ?

Q15. Find THE Vmax and Vmin for 11111 input with binary weighted
DAC givev Vref= 10 V, Rf = 1 kohm, resistance tolerance = 2%. Also find
resolution.
Q16.
Q 16. Find the output voltage from the circuit diagram of a 3-bit R-2R
Ladder DAC when binary input, b2b1b0 = 100 applied to it.
In the above circuit, there exists series and parallel combinations of resistors to the left of
point A with respect to ground. So, we can replace that entire resistor network with a single
resistor having resistance of 2RΩ

The simplified circuit diagram is shown in the following figure −

We can replace the part of the network that is connected to the left of point B with respect to
ground by using a Thevenin’s equivalent circuit. The modified circuit diagram is shown in the
following figure
In the above circuit, there exist a series combination of two resistors. Replace this combination with
a single resistor. The final circuit diagram after simplification is shown in the following figure –