For electrical instrumentation and mesurment

1. CHAPTER THREE
1

2. Introduction
• The output of sensors is usually small or not suitable
to directly process or present
• Thus conditioning or suiting of the signal is
necessary.
• They are circuits that converts the output of a sensor
in to a form more suitable for further processing or
presenting.
• Usually the output is current, voltage or frequency
2

3. Cont’d
• Some of the very commonly used types of
signal conditioning and interfacing circuits are
-Deflection bridge
-Instrumentation Amplifier(INA)
-filters
-V/I, I/V, V/F, F/V
and as interfacing circuits,
- ADC,
- DAC
3

4. Deflection Bridge
• It finds an extensive application in electrical
instrumentation for conditioning the output of
sensors
• typically if the sensor output is variation of
-resistance, inductance and capacitance.
• The deflection bridge has four arms of impedance
designated as
- ZA, ZB, ZN and ZX.
• ZX is the unknown impedance that usually forms
the sensor
4

5. Figure 3.1 Deflection Bridge
5

6. Cont’d
• Initially all impedances values are adjusted so that
voltage at the detector is zero (Vab=0),
• at this moment the bridge is at balanced condition or
null condition which is used a reference point.
• When Vab 0 the bridge is called at off balance or
imbalance accounting for the change or variation of
the sensor.
• The departure from the initial balance or reference is
taken as measure of the non electrical quantity under
interest.
6

7. Cont’d
• The operation of the deflection bridge lies in
the two states or conditions of the bridge
• Balanced: it is a reference (starting from zero)
• Unbalanced: the imbalance accounts for the
measurement of the physical parameter under
going change.
7

8. balanced condition
• Vab=o , which is a reference point(start) for
measurement
• I1Z1=I2Z2
• I1Z4=I2Z3
• By dividing the first equ. by the second equ.
r=Z1/Z4=Z2/Z3 ,where r is called
the arm ratio
8

9. off balance
• Vab0
9

10. Cont’d
• three basic relation ship information’s governing the
operation of the bridge circuit
1. The relation ship among the impedance when the
bridge is at balanced condition
2. The sensitivity of the bridge i.e. the output voltage
per unit change of impedance
3. Loading effect:
• This information will help to design appropriate
deflection bridge that encompasses calibration,
compensation, adjustment of sensitivity and the
provision of adjustment of output voltage to zero.10

11. Types of Deflection Bridge
Deflection bridges are classified depending up on
• Energy source as
-DC or AC
• measurement applications such as ,
- DC bridges( Kelvin double, Wheatstone Bridge)
-AC bridges (Wein, Schering , Maxwell and Owen
bridges)
• The impedances as
-resistive, or reactive (inductive, capacitive) bridges
• number of sensors present on the bridge arm as
- quarter, half and full Deflection Bridge 11

12. DC Bridges
• With a d-c galvanometer used as a detector and
resistive arms is Wheatstone bridge.
• It is almost the standard measuring resistance
based on resistance variations over wide ranges
• usually used to measure resistance values ranging
from 1 ohm to 1mega ohm with accuracy of 0.02 %
• Special difficulties in the measurement of very low
and very high resistance or resistance variations.
12

13. Cont’d
• For low resistance the Kelvin double (modification
of the Wheatstone bridge) can reduce the uncertainty
introduced by the resistance of leads and contacts
• When the resistance being measured is very high,
the bridge galvanometer becomes a relatively
insensitive indicator of unbalanced conditions.
• This is because of the high source impedance that
the bridge presents to the galvanometer
13

14. Design of Resistive Deflection
bridges
• Considering the quarter bridge which has one
sensor
• Quarter Deflection Bridge with R1 as sensor
14

15. Cont’d
• if Imin and Imax are the minimum and
maximum values of measured values of
measured variable and
• RImin and RImax are the corresponding
sensor resistance then
15

16. Cont’d
16

17. Cont’d
• Often the bridge is designed to meet the following
specifics
• VImin=0 , often we need the resistive deflection
bridge balanced at I=Imin
• The electrical power dissipated in the sensor must
not exceed the maximum power
• The non linearity must be with in the limited value ,
the ratio r3/r2 gives in sight in to the non linearity
17

18. Cont’d
• For any input I the output (terminal) voltage
Vab is given by
• Substituting in the above equ.
18

19. Cont’d
19

20. Cont’d
• The degree of non linearity is dependent on r .
• The design should consider achieving high
sensitivity
20

21. AC Bridge
• Detectors commonly used in ac bridges are
-vibration galvanometers (with sensitivities
of 50Hz to 1 KHz)
- earphones (from about 250Hz), and tunable
amplifier detectors (10Hz to 100 KHz).
• For wider bandwidths, CROs with relatively high
sensitivities could also be used.
• various a-c bridges are used in practice under the
broad classification of capacitance and inductance
bridges
21

22. Cont’d
22

23. Quality Factor
23

24. 24

25. Effect of lead wire resistance
• The distance between the strain gauge and the two
lead wires connecting it to the bridge circuit may be
substantial, and
• the wire resistance has a significant impact on the
operation of the circuit.
• As shown in figure the effects of lead wire
resistance is illustrated by addition of resistor
connected series with the strain gauge
25

26. Quarter bridge strain gauge circuit
26

27. Cont’d
• The strain gauge's resistance (Rgauge) is not the
only resistance being measured,
• the wire resistances Rwire1 and Rwire2, being in
series with Rgauge, also contribute to the resistance
of the lower half arm of the bridge, and
• consequently contribute to the voltmeters indication.
• this effect cannot be completely eliminated
• it can be minimized with the addition of a third wire,
connecting the right side of the voltmeter directly to
the upper wire of the strain gauge
27

28. Cont’d
• Thus the resistance of the top wire (Rwire1) is
“bypassed" and the voltmeter is connected
directly to the top terminal of the strain gauge,
leaving only the lower wire's resistance (Rwire2)
to contribute any stray resistance in series with
the gauge.
• Practically the third wire carries no current (due
to the voltmeter's extremely high internal
resistance) and
• its resistance will not drop any substantial
amount of voltage 28

29. Cont’d
29

30. Compensation for temperature
• Another kind of measurement error associated with
the strain measurement is due to
environmental effects such as temperature.
• An unfortunate characteristic of strain gauges
is that its resistance change with changes in
temperature.
• This is a property common to all conductors, some
more than others
• Thus, the quarter-bridge circuit works as a
thermometer just as well as it does a strain indicator.
30

31. Cont’d
• By using a "dummy" strain gauge in place of
R2, the effects of temperature change can be
cancelled.
• As both elements of the rheostat arm will same
resistance change in response to temperature
changes the effect of temperature can be
avoided.
• For example, Figure bellow illustrates a strain
gauge configuration where one gauge is active
(RG + ΔR), and a second gauge is placed
transverse to the applied strain
31

32. cont’d
• the strain has little effect on the second gauge,
called the dummy gauge.
• However, any changes in temperature will affect
both gauges in the same way.
• Because the temperature changes are identical in
the two gauges, the ratio of their resistance does
not change,
• the voltage Vo does not change, and the effects of
the temperature change are minimized.
• Resistors R1 and R3 are of equal resistance value
and the strain gauges are identical
32

33. Cont’d
33

34. Increasing the sensitivity and
linearity
• one can double the sensitivity of the bridge to
strain by making both gauges active, although in
different directions
• one bridge mounted in tension (RG + ΔR) and
the other mounted in compression (RG - ΔR).
• This is called half-bridge configuration
• It yields an output voltage that is linear and
approximately doubles the output of the quarter-
bridge circuit
34

35. Half bridge strain gauge circuit
35

36. Cont’d
• both strain gauges will either increase or
decrease resistance by the same proportion in
response to changes in temperature
• the effects of temperature change remain
canceled and
• the circuit will suffer minimal temperature-
induced measurement error:
36

37. Full bridge strain gauge circuit
• it may be advantageous to make all four
elements of the bridge "active" for even
greater sensitivity.
• This is called a full-bridge circuit
37

38. Cont’d
• Vo/Vi=-GF
• Both half-bridge and full-bridge configurations
grant greater sensitivity over the quarter bridge
circuit, but
• often it is not possible to bond complementary
pairs of strain gauges to the test specimen.
• Thus, the quarter-bridge circuit is frequently used
in strain measurement systems.
• When possible, the full-bridge configuration is
the best to use.
38

39. Cont’d
• This is true not only because it is more sensitive
than the others, but
• because it is linear while the others are not
• Quarter-bridge and half-bridge circuits provide an
output (imbalance) signal that is only
approximately proportional to applied strain
gauge force.
• Linearity of these bridge circuits is best when the
amount of resistance change due to applied force
is very small compared to the nominal resistance
of the gauge(s) 39

40. Cont’d
• With a full-bridge, however, the output voltage is
directly proportional to applied force,
• with no approximation (provided that the change
in resistance caused by the applied force is equal
for all four strain gauges).
• Strain gauges may be purchased as complete
units, with both strain gauge elements and bridge
resistors in one housing, sealed and encapsulated
for protection from the elements, and equipped
with mechanical fastening points for attachment
to a machine or structure.
• Such a package is typically called a load cell
40

41. Amplifiers
• Since most of the output from sensors is low, often
it is necessary to amplify them before they are used
for further processing, indication or recording.
• An Amplifier is an electronic device or group of
devices used to increase the size of a voltage or
current signal without changing the signals basic
characteristics.
41

42. Operational amplifier (OPAMP)
• Operational amplifiers (OPAMPs) are special
types of amplifiers which essential component of
both practical and precision instruments.
• Their characteristics make opamp to find wide
application in instruments as signal conditioning
and signal conversion circuits such as ;
• Instrumentation Amplifier(INA),
• filters,oscillator,integrator,differentiator,
V/I,I/V,V/F,F/V,ADC,DAC
42

43. Cont’d
• Opamp has two inputs indicated by – and + sign
which stands for inverting and non inverting inputs
Schematic diagram of operational amplifier (OPAMP)
43

44. Characteristics of ideal OPAMP
• high input amplification
• high input impedance
• low output impedance
• low offset voltage
• low drift
• Vo= A (v2- v1)
• has same amplification (A) for both inputs thus the
Vo is the difference of the inputs meaning if there is
a common voltage(Vc) to both input terminals then
it will be cancelled
44

45. Cont’d
• in practice there is amplification called as common
gain for common voltages, thus the output is
Vo= A (v2- v1) +AcVc
• The degree of deviation from the ideal opamp is
specified by common mode rejection ratio (CMRR)
• CMRR = A/Ac= |Adm|/|Acm|
• in any case the use of amplifiers with high CMRR
ratio is helpful in reducing errors.
45

46. Common opamp circuit configurations
• Voltage comparator
• Inverting amplifier
• Non inverting amplifier
• Summing amplifier
• Voltage follower
• Differential amplifier
• Integrating amplifier
• Differentiating amplifier,
• V/I,I/V,V/F,F/V
46

47. Voltage comparator
• This is used for comparing input voltages, it is a
differential amplifier
• If v2>v1 the output voltage is positive
• If v2<v1the output voltage is negative
• If v2=v1, then the output voltage is zero
47

48. Inverting amplifier
• The out put is an inverted (opposite polarity)
form of the input voltage
Vo=-V1[R2/R1]
48

49. Non inverting amplifier
• The out put is the non inverted form of the
input
Vo=V1[1+R2/R1]
49

50. Summing amplifier
• The output is the sum of the input voltages
• Vo=-V1R3/R1-V2R3/R2
50

51. Voltage follower
• The output is the same as the input. V0=v1
• The advantage of the circuit lies in high input
impedance avoiding loading effect that may
occur;
• it is also referred to as buffer circuit.
• v
51

52. Differential amplifier
• The output is used to amplify the difference of
inputs
• Vo=R3/R1(V2-V1)
52

53. Integrating amplifier
• Vo=-V1xt/RC
53

54. Differentiating amplifier
• Vo=-V1xCxR/t
54

55. Current to voltage converter
• V0=-IR
55

56. Voltage to current converter
• I’=V1/R1
56

57. Instrumentation Amplifier (INA)
• An instrumentation amplifier (INA) is specially
designed amplifier to have
-differential gain
- high input impedance, and
- high CMRR ratio
• Basically it is buffered a differential amplifier
so as to have high input impedance.
57

58. Cont’d
58

59. Cont’d
• The output voltage can be easily found by
analyzing the circuit as shown
59

60. The voltage difference
60

61. Cont’d
• The front end of the instrumentation amplifier is a
difference amplifier that can be analyzed by
superposition theorem i.e. the output is due to an
inverting amplifier and non-inverting amplifier.
• The inverting amplifier produces
Voi=-Vo1(R4/R3)
• The non-inverting amplifier produces
Von=Vo2[R4/(R3+R4)][1+R4/R3]
• The output is the sum of the outputs produced by
each input.
61

62. Cont’d
62

63. Cont’d
• The INA is having additional buffer amplifiers to
the inputs so as to make Rin infinity at
• both V1 and V2 especially as compare to the
front differential amplifier.
• In practice these differential signals typically
emanate from sensors such as resistive bridges
or thermocouples
63

64. Interfacing circuits
• The (ADC) and (DAC) makes important part
of instruments as interfacing analog and digital
electronic equipments such as
-computer
- data acquisition systems
- data loggers.
64

65. Cont’d
65

66. Cont’d
• The purpose of ADC is to generate a train of pulses
proportional to the analog input.
• It electronically translates analog signals into digital
(binary) quantities.
• While a DAC performs the conversion of digital
signal in to analog electrical signal such as voltage
output.
• Together, they are often used in digital systems to
provide complete interface with analog sensors and
output devices for control systems.
66

67. Digital to Analog Conversion (DAC)
• The two most commonly used types of DAC are
a) R/2nR DAC/ binary-weighted-input DAC
b) R/2R ladder DAC
67

68. The R/2nR DAC/ binary-weighted-input
• The R/2nR DAC circuit, known as the binary-
weighted-input DAC, is an inverting summer op-
amp circuit which has the input resistor values
set at multiple powers of two:
• R, 2R, and 4R…. 2nR and supplied by voltages V
68

69. Cont’d
69
• The output voltage is given by

70. Cont’d
• Usually Vo=V1 =V2= ....Vn, then for n binary bit
inputs the analog output voltage is given
• If we drive the inputs of the circuit with digital
gates so that each input is either 0 volts or full
supply voltage, the output voltage will be an
analog representation of the binary value of these
three bits
70

71. Cont’d
• Each input voltage has exactly half the effect on
the output as the voltage before it.
• In other words, input voltage V1 has a 1:1 effect
on the output voltage (gain of 1),
• while input voltage V2 has half that much effect
on the output (a gain of 1/2), and
• V3 half of that (a gain of 1/4).
• These ratios are the same ratios corresponding
to weights in the binary numeration system.
71

72. The R/2R DAC
• An alternative to the binary-weighted- input DAC
• uses fewer unique resistor values
• disadvantage of the former design was requirement
of several different precise input resistor values one
unique value per binary input bit.
• However in “ladder” type of DAC we require only
two resistor values of R and 2R
72

73. Cont’d
73

74. Cont’d
• One leg of the DAC is grounded while all the
legs of the DAC are connected to the switches
having 1 and 0 states meaning:
• When the corresponding switch is connected
to VREF, bit =1
• When the corresponding switch is connected
to GND, bit =0
74

75. Analog to Digital Converter (ADC)
• Connecting digital circuitry to sensor devices is
simple if the sensor devices are inherently digital
themselves.
• Switches, relays, and encoders are easily
interfaced with gate circuits due to the on/off
nature of their signals.
• However, when analog devices are involved,
interfacing becomes much more complex.
• What is needed is a way to electronically
translate analog signals into digital (binary)
quantities
75

76. Analog to digital converter (ADC)
76

77. types of ADC
1. Flash or parallel ADC
2. Counter ADC
a. Digital ramp or stair step-ramp ADC
b. Successive approximation ADC
c. Tracking ADC
3. Slope integrating ADC
a. single slope integrating ADC
b. dual slope integrating ADC
77

78. Flash ADC
78

79. Cont’d
• The flash is also called the parallel ADC, basically
comprises of comparator and priority encoder
• The operation is :analog input signal is compared
with a reference voltage by the comparator, and
the result is encoded by the priority encoder,
which produces digital outputs .
• The flash ADC comprises of 2N-1 comparators for
N bit binary outputs.
79

80. Cont’d
• The flash ADC is simple in construction and
operation
• it is most efficient of the ADC technologies in
terms of speed, being limited only in
comparator and gate propagation delays (e.g.,
very fast 8-bit ADCs capable of 20 million
conversions/sec).
• Unfortunately, for large N it is very expensive
and component intensive
80

81. Counter ADC
• These are commonly used types of ADC
• basically comprises of (DAC), counter and
comparator for conversion process.
• Their operation is by means of :the counter
connected to DAC which produces analog signal
that can be compared with analog signal to be
digitized and when they become equal, the counter
will stop counting and its current binary output is
the digitized form of the analog signal.
• The types of counter ADC are digital or stair step
ramp, successive approximation, and tracking ADC
81

82. Cont’d
• As the counter counts up with each clock pulse, the
DAC outputs a slightly higher (more positive)
voltage
• This voltage is compared against the input voltage
by the comparator.
• If the input voltage is greater than the DAC output,
the comparator's output will be high and the
counter will continue counting normally.
• If the input voltage is equal or lower than the DAC
output, the comparator's output will be low and the
counter will be reset and the latch will execute a
binary output 82

83. Cont’d
• The effect of this circuit is to produce a DAC
output that ramps up to whatever level the
analog input signal is at, output the binary
number corresponding to that level, and start
over again.
• This ADC is relatively slow since conversion
time could be up to 2N, where N is the
resolution of the ADC
83

84. Performance characteristics of ADC ckts
• The most important consideration in selecting
or designing ADC is their performance
characteristics which is described by
-resolution
- Sample frequency (or conversion rate)
-step recovery
84

85. Resolution
• Resolution is the number of binary bits output by
the converter.
• An ADC with a 10-bit output can represent up to
1024 unique conditions of signal measurement.
• Over the range of measurement from 0% to 100%,
there will be exactly 1024 unique binary numbers
output by the converter (from 0000000000 to
1111111111, inclusive).
• Resolution is very important in instrument
employing ADC such as data acquisition systems
85

86. Sample frequency or conversion rate
• the speed at which the converter outputs a
new binary number.
• Like resolution, this consideration is linked to
the specific application of the ADC.
86

87. Step recovery
• The step recovery is a measure of how quickly an
ADC changes its output to match a large, sudden
change in the analog input.
• In some converter technologies especially, step
recovery is a serious limitation.
• An ideal ADC has a great many bits for very fine
resolution, samples at lightning-fast speeds, and
recovers from steps instantly.
87