Download 2. digital instruments - ELECTRONICS AND INSTRUMENTATION

2. DIGITAL INSTRUMENTS
INTRODUCTION:
The digital instruments display measurement of electrical parameters like voltage and current
as discrete numerals instead of pointer deflection on a continuous scale as in analog devices.
Numerical read out is advantageous in many applications because it reduces human error, parallax
error, increases reading speed and often provides outputs in digital form suitable for further
processing or recording.
Digital instruments, particularly digital voltmeters or multimeters, are used to measure
analog quantity. It is, therefore, necessary to convert the analog signal to an equivalent digital signal.
So, analog-to-digital converters (A/D converters) are also used as a main element of a digital
instrument. The basic building block of a digital instrument is shown in fig.
Analog
To
Digital
Converter
Signal
Processing
Display
Fig (a): Building Block of a Digital Instrument
The display block may be analog or digital in nature. If an analog readout is desired, it
becomes necessary to include a stage involving digital to analog conversion.
2.1 ADVANTAGES OF DIGITAL INSTRUMENTS OVER ORDINARY INSTRUMENTS:
1. It reduces human reading & interpolation errors.
2. It eliminates parallax error.
3. Increases reading speed.
4. Digital instruments have high accuracy up to ± 0.003%.
5. They have high resolution.
6. They possess good reliability and repeatability.
7. Loading effect is eliminated due to their high input impedance.
8. They have high sensitivity.
9. They have wide range of frequency response.
10. They have wide range of frequency measurement.
11. They are portable due to their small size.
12. The digital instruments also have greater speed.
13. They have very low power consumption.
14. It provides output in digital form suitable for further processing and recording.
15. They have high input range from ±1.000000 to ± 1,000.000 V.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 1
2. DIGITAL INSTRUMENTS
2.2 DIGITAI VOLTMETERS (DVM) :
Digital voltmeters are measuring instruments that convert analog voltage signals into a digital
or numeric readout. This digital readout can be displayed on the front panel and also used as an
electrical digital output signal.
The digital voltmeter (DVM) displays ac and dc voltages as discrete numbers, rather than as a
pointer on a continuous scale as analog voltmeter. A numerical readout is advantageous because it
reduces human error, eliminates parallax error, increases reading speed and often output in digital
form suitable for further processing and recording.
The DVM is a versatile and accurate instrument that can be used in many laboratory
measurement applications. With the development of IC modules, the size, power requirements and
cost of DVMs have been reduced, so that DVM compete with analog voltmeters in portability and
price.
Digital voltmeters can be classified according to the following broad categories
(a) Ramp-type DVM
(b) Dual slope integrating type DVM (Voltage to time conversion)
(c) Integrating type DVM (Voltage to frequency conversion)
(d) Staircase - Ramp DVM
(e) Successive - Approximation DVM
(f) Continuous-Balance type DVM.
2.2 (a) Ramp-Type DVM:
Operating Principle: The operating principle in the Ramp type DVM is based on the measurement
of the time it takes for a linear ramp voltage to, rise from 0V to the level of the input voltage, or to
decrease from the level of the input voltage to zero. This time interval is measured with an electronic
time interval counter and the count is displayed as a number of digits on electronic indicator.
The block diagram of a Ramp type DVM is as shown in Fig 1 .
FIG 1: Block Diagram of Ramp Type DVM
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 2
2. DIGITAL INSTRUMENTS
Description: The Ramp type DVM consists of a ranging & attenuator which is used to select the
required range of measurement. A ramp generator is used to generate the ramp voltage. An input
comparator continuously compares the dc input voltage with the ramp voltage. The ground
comparator compares the ramp voltage with the ground voltage i.e., 0 V. A counter is used to count
the number of pulses and the read out gives the digital display.
Working :
At the start of the measurement cycle a ramp voltage is initiated; this voltage can be positive going or
negative going. This -ve going ramp is illustrated in Fig. 2.
FIG 2: Voltage to Time Conversion
This negative going ramp is continuously compared with the unknown input voltage. At the
instant that the ramp voltage equals to unknown voltage, the comparator generates the pulse which
opens the gate. The ramp voltage continues to decrease with time until it finally reaches 0 V and a
second comparator generates an output pulse which closes the gate.
An oscillator generates clock pulses which are allowed to pass through the gate to a number of
decade counting units which totalize the number of pulses passed through the gate. The decimal
number, displayed by the indicator tubes is a measure of the magnitude of the input voltage. The
sampled rate multivibrator determines the rate at which the measurement cycles are initiated. The
sample rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage
.At the same time, a reset pulse is generated which resets the counter to the zero state.
Advantages and Disadvantages:
The ramp technique circuit is easy to design and its cost is low. Also, the output pulse can be
transmitted over long feeder lines. However, the single ramp requires excellent characteristics
regarding linearity of the ramp and time measurement. Large errors are possible when noise is
superimposed on the input signal. Input filters are usually required with this type of converter.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 3
2. DIGITAL INSTRUMENTS
2.2 (b) Successive Approximation Type Digital Voltmeter:
The successive approximation type DVM works on the principle of balancing the weights as
in a simple balance. To understand the concept clearly, let us consider we want to measure the weight
of some unknown quantity of sugar. What do we do? First we approximate the weight of sugar to
some known weight, If the weight of sugar is more than known weight, then we add some more
weights to the known weight. If it is less, then we replace the weight with a lesser value. This process
is repeated until the pointer balances the two weights. The successive approximation type DVM uses
the same principle.
The basic block diagram of a successive approximation type DVM is as shown in the Fig:3
FIG 3 : Successive Approximation Type DVM
Description : This DVM consists of an input attenuator for selecting the desired range of input
voltage and also to attenuate any noise in the given voltage. This selected input is applied to the
comparator through a sample and hold circuit. The successive approximation register (SAR) receives
its 8 bit input from the ring counter after each clock pulse. This Input is applied to the Digital to
Analog converter which converts the digital data into Analog voltage. This voltage is applied as
second input to the comparator. The o/p of the AND gate goes high when there is a positive o/p at the
comparator. Finally, the digital output is taken out from the successive approximation register with
input voltages other than dc; the input level changes during digitization and decision made during
conversion are not consistent. To avoid this error, a sample and hold circuit is used and placed in the
input directly following the input attenuator. This digital voltmeter is capable of 1000 readings per
second.
Working: When the start pulse signal activates the control circuit, the SAR is cleared. Therefore,
the output of the SAR is 0000 0000.V OUT of the D/A converter is 0. Now if
V in > V out, the comparator output is positive. During the first clock pulse, the control circuit sets D 7,
to 1 and V out jumps to the half of reference voltage. The SAR o/p is 10000000. If V out is greater than
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 4
2. DIGITAL INSTRUMENTS
V in, the comparator o/p is negative and the control circuit resets D 7. However, if V in is greater than
V out, the comparator output is positive and the control circuit keeps D 7 set. Similarly the reset of the
bits beginning from D 7 to D0 are set and tested. Therefore the measurement is completed in eight
clock pulses.
Example: Suppose the converter can measure a maximum of 5 V. i.e. 5 V corresponds to the
maximum count of 11111111. If the test voltage V in = 1V the following steps will take place in the
measurement.
FIG 4 : Various Output Levels for Each Bit ( 8-Bit Shows the Voltage Level Very
Nearly Equal to 1V)
2.2 (c) DUAL SLOPE INTEGRATING TYPE DVM : (VOLTAGE TO TIME CONVERSION)
In the ramp techniques, superimposed noise can causes large errors. In the dual ramp
technique noise is averaged out by positive and negative ramp using the process of integration
As shown in figure 8(b) the input voltage ‘ei ‘ is integrated with the slope of the integrator output
proportional to the test input. After a fixed time equal to t1 the input voltage is disconnected and the
input voltage is connected to negative voltage -er the integrator output will have a negative slope
which is constant and proportional to the magnitude of the input voltage.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 5
2. DIGITAL INSTRUMENTS
FIG 8 (a) : Block Diagram of Dual Slope Integrator DVM
FIG 8 (b) : Waveforms of Dual Slope Integrator
At the start a pulse reset the counter and the F/F output to logic ‘0’. Si is closed and Sr is open.
The capacitor begins to charge as soon as the integrator output exceeds zero the comparator output
voltage changes state, which opens the gate so that the oscillator clock pulses are fed to the
counter.(when the ramp voltage starts, the comparator goes to state 1, the gate opens and clock pulse
drives the counter)when the counter reaches the maximum count, i.e. the counter is made to run for a
time ‘t1’ in this case 9999, on the next clock pulse all the digits go to 0000 and the counter activates
the F/F to logic level ‘1’. This activates the switch drive, ei is disconnected and -er is connected to the
integrator. The integrator output will have negative slope which is constant, i.e. integrator output is
now decreases linearly to zero volts. Comparator output state changes again and locks the gate. The
discharge time t2 is now proportional to the input voltage. The counter indicates the count during t2.
When the slope of the integrator reaches the zero, the comparator switches to state ‘0’ and the gate
closes and the capacitor is now discharge with constant slope. As soon as the eo is zero the counter is
stopped. The pulses counted thus has the directly relation with the input voltage.
During charging
eo= -1/ RC ∫ot 1 eidt = -ei t 1/ RC
(eq-1)
During discharging ( For analysis reference source value is taken as -er)
eo = 1/ RC ∫ot 2 –er dt = -er t 2 / RC
(eq-2)
Subtracting Eqs 2 from 1 we have
eo - eo = -er t 2 /RC – (-ei t 1/ RC)
0 = -er t 2 /RC – (-ei t 1/ RC)
er t 2 /RC = ei t 1/ RC
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 6
2. DIGITAL INSTRUMENTS
There fore
ei = er (t 2/ t 1)
(eq-3)
If the oscillator period equals T and the digital counter indicates n 1and n 2 counts respectively.
ei = (n 2 T/ n 1 T) er i.e. ei = (n 2 / n 1) er
Now, n 1 and er are constants. Let
K 1= er/ n 1.Then ei =K1 n 2
(eq-4)
From Eq -3 it is evident that the accuracy of the measured voltage is independent of the
integrator time constant The times t 1 and t 2 are measured by the count of the clock given by the
numbers n 1 and n 2 respectively. The clock oscillator period equals T and if n 1and er are constants,
then Eq-4 indicates that the accuracy of the method is also independent of the oscillator frequency.
Advantages & Disadvantages of Dual Slope Technique :
(a) The dual slope technique has excellent noise rejection as the noise is averaged over in
the process of integration.
(b) The speed and accuracy can be easily varied according to demands of the
measurement situation. The speed is high.
(c) The only source of error is the reference voltage. Hence the system is suitable for
accurate measurements. An accuracy of ± 0.05% in 100 ms is available. Accuracy is
independent of oscillator frequency.
d) Filters are not required
e) Highly stable.
Comparison with ramp type voltmeter.
S.NO.
Ramp Type
Dual Slope
1
Large errors are possible when noise Has excellent noise rejection because
is superimposed on the input signal.
noise and superimposed ac are
averaged out in the process of
integration.
2
Circuit complexity is low
3
4
5
6
Circuit complexity is moderate.
Input filters are required
Filters not required
Low accuracy and accuracy depends
on the stability of the oscillator and Accuracy of this DVM is high.
Accuracy is independent of oscillator
the linearity of the ramp slope
frequency.
Poor Stability
High Stable
Operating speed slow
Operating speed high
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 7
2. DIGITAL INSTRUMENTS
Comparison of Various types of DVM’s :
System
Accuracy
Input Impedance
Speed
Staircase ramp
High
Not constant (Low to high)
Medium
Dual –Slope integration
High
Very high
Medium
Successive Approximation
High
Not constant(Low to high)
High
2.3 GENERAL SPECIFICATIONS OF DIGITAL VOLTMETERS :
I. Display: 3-1/2 digits, LCD.
2. Input Range: from ± 1.000 V to ± 1000 V, with automatic range selection and overload
indication.
3. Over-Range Indication: Only (1) or (-1) displayed at the MSB position.
4. Absolute Accurcuracy : as high as ±0.005 percent of the reading.
5. Stability : Short-term, 0.002% of the reading for a 24 period; long-term. 0.008% of the
reading for a 6-month period.
6. Resolution : 1 part in 10 6 (1μV can be read on the 1V input range)
7. Input characieristics : Input resistance typically 10 MΩ; input capacitance typically
40 PF.
8.Calibration : Internal calibration standard allows calibration independent of the
measuring circuit.
9. Output signals : Print command allows output to printer; BCD output for digital
processing or recording.
10. Zero adjustment: Automatic
11. Functions : DC volts, AC volts, DC amps AC amps, ohms, diode test.
12. Manual : Switch selection as desired.
13. Temperature: Operating 0°C — 60°C, 70% RH (Relative humidity)
14. Polarity : AUTO negative polarity indication .
15. Low Battery: B mark on LCD readout.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 8
2. DIGITAL INSTRUMENTS
3 ½ - DIGIT
The number of digit positions used in a digital meter determines the resolution. Hence a 3
digit display on a DVM for a 0-1 V range will indicate values from 0-999 mV. Normally , a fourth
digit capable of indicating 0 or 1 (hence called a Half Digit) is placed to the left. This permits the
digital meter to read values above 999 up to 1999, to give overlap between ranges for convenience, a
process called over-ranging. This type of display is called a 3½ digit display, shown in Fig : 9.
FIG 9 : 3 ½ Digit Display
5,9. RESOLUTION AND SENSITIVITY OF OIGITAL METERS
Resolution
If n = number of full digits, then resolution (R) is 1/10 n .
The resolution of a DVM is determined by the number of full or active digits
used,
I f n=3, R=1/10 n =1/10 3 =0.001 or 0.1%
Sensitivity of Digital Meters
Sensitivity is the smallest change in input which a digital meter is able to detect. Hence, it is the full
scale value of the lowest voltage range multiplied by the meter’s resolution.
Sensitivity S = (fs) min x R
where (fs) min = lowest full scale of the meter
R = resolution expressed as decimal
.Example 1: What is the resolution of a 3½ digit display on I V and 10 V ranges?
Solution Number of full digits is 3, Therefore, resolution is 1/10 n where
n =3. Resolution R =1/10 n = 1/103 = 0.001
Hence the meter cannot distinguish between values that differ from each other by less than 0.001 of
full scale.
For full scale range reading of 1V, the resolution is 1 x 0.001 = 0.001 V.
For full scale reading of 10 V range, the resolution is 10 V x 0.001 = 0.01 V.
Hence on 10 V scale, the meter cannot distinguish between readings that differ by less than 0.01 V.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 9
2. DIGITAL INSTRUMENTS
2.4 DIGITAL MULTIMETER’S:
The three major classes of digital meters are panel meters, bench type meters and system
meters.
All digital meters employ same kind of analog to digital (often dual slope integrating type)
and have a visible readout display at the converter output.
Panel meters are usually placed at one location (and perhaps even a fixed range). while bench
meters and system meters are often multimeters, i.e. they can read ac and dc voltage currents and
resistances over several ranges. Bench meters are intended mainly for stand alone operation and
visual operation reading, while system meters provide at least an electrical binary coded decimal
output (in parallel with the usual display), and perhaps sophisticated interconnection and control
capabilities, or even microprocessor based computing power.
The basic circuit shown in Fig10:(a) is always a dc voltmeter. Current is converted to voltage
by passing it through a precision low shunt resistance while alternating current is converted into dc by
employing rectifiers and filters. For resistance measurement, the meter includes a precision low
current source that is applied across the unknown resistance; again this gives a dc voltage which is
digitized and readout as ohms.
FIG 10 : (a) Digital Multimeter
The basic digital multimeter (DMM) is made up of several A/D converters, circuitry for
counting and an attenuation circuit. A basic block diagram of a DMM is shown in Fig10 (b). The
current to voltage converter shown in the block diagram of Fig. 10 (b) can be implemented with the
circuit shown in fig 10 (c).
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 10
2. DIGITAL INSTRUMENTS
FIG 11 : (b) Block Diagram of a Basic Multimeter
The current to be measured is applied to the summing junction (∑ i) at the input of the opamp.
Since the current at the input of the amplifier is close to zero because of the very high input
impedance of the amplifier, the current I R is very nearly equal to I i, the current I R causes a voltage
drop which is proportional to the current, to be developed across the resistors. This voltage drop is the
input to the A/D converter, thereby providing a reading that is proportional to The unknown current.
FIG 13 : (c) Current to Voltage Converter
Resistance is measured by passing a known current, from a constant current source, through
an unknown resistance. The voltage drop across the resistor is applied to the A/D converter, thereby
producing an indication of the value of the unknown resistance .
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 11
2. DIGITAL INSTRUMENTS
2.5 SPECIFICATIONS OF DIGITAL MULTIMETER:
1. Display :3 ½ digit LCD with a maximum reading of 1999.
2. Polarity : Automatic, (-)negative polarity indication.
3. Over range indication : (OL) or (-OL) is displayed.
4. Operating Environment : 0 0 c to 50 0 c at <70% R.H. (Relative Humidity)
5. Input Impedance :10MΩ in all DCV and ACV ranges.
6. Power requirements :9V Battery.
7. Dimensions: 91 mm (w) Χ 170 mm (L) Χ40 mm (H)
8.Weight : 330 gms
9. DC volts :
Ranges : 200 mv,2V,20V,200V, 1000V
Resolution :100 μV
Accuracy : ± 0.5% of reading
10. AC volts :
Ranges : 200mV,2V, 20V,200V,750V
Accuracy : ± 1% of reading (40-60Hz)
Resolution : 100 μV
11. DC current :
Ranges : 200 μA,2mA,20 mA,200mA,2A,10A.
Accuracy : ±0.5% of reading
12 AC current :
Ranges :2 mA,200mA,10 A
Accuracy : ± 1% of reading
13. Resistance :
Ranges : 200Ω,2KΩ,20KΩ,200KΩ,2MΩ,20MΩ
Accuracy : ±0.5% of reading
14. Diode test :
Test Current 0.8 mA ±0.3mA
Open current volts :3.0 V
15.Transistor,h f e :
Ranges :0-1000
Base current : 10 μ A DC approx. (V c e=3.0V DC)
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 12
2. DIGITAL INSTRUMENTS
2.6 DIGITAL FREQUENCY METER :
Principle of Operation
The signal waveform is converted to trigger pulses and applied continuously to an AND gate,
as shown in Fig 14. A pulse of 1s is applied to the other terminal, and the number of pulses counted
during this period indicates the frequency.
FIG 14 : Principle Of Digital Frequency Measurement
The signal whose frequency is to be measured is converted into a train of pulses, one pulse for
each cycle of the signal. The number of pulses occurring in a definite interval of time is then counted
by an electronic counter. Since each pulse represents the cycle of the unknown signal, the number of
counts is a direct indication of the frequency of the signal. Since electronic counters have a high
speed of operation, high frequency signals can be measured.
Basic Circuit of a Digital Frequency Meter :
The block diagram of a basic circuit of a digital frequency meter is shown in Fig 15.
FIG 15 : Basic Circuit of a Digital Frequency Meter
The signal may be amplified before being applied to the Schmitt trigger. The Schmitt trigger
converts the input signal into a square wave with fast rise and fall times, which is then differentiated
and clipped. As a result, the output from the Schmitt trigger is a train of pulses one pulse for each
cycle of the signal.
The output pulses from the Schmitt trigger are fed to a START/STOP gate. When this gate is
enabled, the input pulses pass through this gate and are fed directly to the electronic counter, which
counts the number of pulses.
When this gate is disabled, the counter stops counting the incoming pulses. The counter displays the
number of pulses that have passed through it in the time interval between start and stop. If this
interval is known, the unknown frequency can be measured.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 13
2. DIGITAL INSTRUMENTS
The block diagram of a digital frequency meter is shown in fig 16.
FIG 16 : Diagram of a Frequency Meter
The input signal is amplified and converted to a square wave by a Schmitt trigger circuit. In
this diagram, the square wave is differentiated and clipped to produce a train of pulses, each pulses
separated by the period of the input signal. The time base selector output is obtained from an
oscillator and is similarly converted into positive pulses.
The first pulse activates the control Flip-Flop, this gate control F/F provides an enable signal
to the AND gate. The trigger pulses of the input signals are allowed to pass through the gate for a
selected time period and counted. The second pulse from the decade frequency divider changes the
state of control F/F and removes the enable signal from the AND gate, thereby closing it. The decimal
counter and display unit output corresponds to the number of input. Pulses received during a precise
time interval; hence the counter display corresponds to the frequency.
The frequency of the input signal‘s computed as
F=N / t
Where,
F= Frequency of the input signal
N = Number of pulses counted
t = Duration of gate pulses
In some applications it is desirable to measure the timeperiod of the signal,the timeperiod is
computed as
T=N/f
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 14
2. DIGITAL INSTRUMENTS
Where,
T = Time period of input signal.
N = Number of pulses countyed,
f = Clock frequency.
2.7 SPECIFICATIONs OF DIGITAL FREQUENCY METER :
The front panel of digital frequency counter is shown in fig 17 and also its specifications are shown
below :
FIG 17 : Front Panel of Digital Frequency Meter
The specifications of digital frequency meter are listed below.
1.
Frequency range
: 10 Hz - 300 MHz (max)
2.
No. of digits
: 5 to 8 digits (LED display)
3.
Response time
: 0.2 seconds
4.
Input sensitivity
: kHz range
10Hz- I MHz,50mVrms
MHz range
1MHz-300MHz, 40mVrms
5.
Resolution
: 0.1 Hz (at 10 sec gate time)
100 Hz (at 0.01 sec gate time)
6.
Time Base
: 10 MHz (Crystal osc ±10 ppm)
7.
Power supply
: 230 V 50Hz a.c
8.
Impedance
: 3M Ω
9.
Dimensions
: 250 mm x 170mm x 70mm
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 15
2. DIGITAL INSTRUMENTS
2.8 BLOCK DIAGRAM OF DIGITAL LCR METER (DIGITAL IMPEDANCE METER ) :
FIG 18 : Block Diagram of Digital LCR Meter
The above block diagram of digital LCR meter, explains,
1. Switch circuit
2. Conversion circuits
3. Phase sensitive detector
4. A/D converter
5. Decade counter
6. Digital read out
1. Switching Circuit:
(a) Manual Switch
(b)Automatic/Inside Semiconducting Switch
For selecting measurement of Inductance (L), capacitance (C), resistance (R) finding unknown
values of above components, front panel controls of LCR meter has manual/ Automatic switches,
whose can switch for required flow for measuring unknown values switch position 1, 2, 3 meant for
measuring unknown inductance (L),capacitance‘C’, resistance (R), as indicated in above block
diagram (Fig 18).
2. Conversion Circuit:
(a) For measurement of unknown inductance
(b)For measurement of unknown capacitance.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 16
2. DIGITAL INSTRUMENTS
FIG 19 : Conversion Circuit of Inductive Impedance into voltage
Conversion circuit is consisting of Op-amp voltage given to non-inverting terminal and
measure the current in unknown Inductance’ L’, (i.e.) the current is ‘I ‘ through resistor gives voltage
developed across ‘R’.
Input voltage = 1.6 V
Resistor = 1 k Ω
I=1.6V/1K Ω = 1.6mA
Then
V L = I . X L = I . (2πfL) =1.6 x 2π x 1kHz x assume some value
ie, 100 mH
= 1 V(rms)
If
L= Assumed value= 200 mH
Then
V L = 1.6 x 2π x 1kHz x 200mH
= 3V
i.e., we conclude that voltage developed across ‘L’ is directly proportional to the
Inductive impedance.
For Measuring Capacitance Value :
FIG 20 : Conversion Circuit for Capacitive Impedance into Voltage
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 17
2. DIGITAL INSTRUMENTS
Input voltage is developed across the capacitor and the output voltage is measured across the resistor
I = V / Xc
VR=IR
V = Input voltage = 1.6 V (Vrms)
f = 1KHz
R = 1 kΩ
Assume ‘C’ value is 0.1 μF
Then
I =V/Xc = V(2πfc) = 1.6x 2π x1kHz x 0.1μF
= 1mA
V = IR =1mA x1 kΩ = 1V (rms)
If we assume ‘C’ value as 0.3 μF
Then
I= 3mA,
V R = 3V
Which indicates voltage developed across ‘R’ is directly proportional to the capacitive impedance.
3. Phase Sensitive Detector:
• In case of ‘L’ measurement. it is a transistorized shifter, or Op-amp shifter circuit, is employed to
solve the Inductor voltage into
(a) Quadrature
(b)
In-phase voltages
i.e., Rs+ jω ‘Ls’ form, and fed to digital measuring circuit for display series equivalent circuit
inductance Ls, which in turn dissipation factor ‘D’ and Q-factor (Q =1/D) values.
• In case of ‘C’ measurement, the phase sensitive detector gives the resistor voltages into quadrature
and in-phase components, proportional to the capacitive current. The displayed capacitance
measurement is that of parallel equivalent circuit (Cp), and also ‘D’ dissipation factor and Q-factor
(Q=1/D).
For Measurement of Resistance using LCR Meter :
Taking Op-amp as current to voltage converter, known current is passing through constant
current source and in feed back path unknown resistor ‘R’ is connected and observe the voltage drop
across the unknown resistor, the voltage drop across the resistor is applied to A/D converter, which
converts. The analog data values into digital values and standard values are maintained to get the
final output as indication of the value of the unknown resistance ‘R’.
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 18
2. DIGITAL INSTRUMENTS
FIG 21 : Circuit Diagram of Current to Voltage Conversion
The digital RCL meter shown in Fig : 22 can measure inductance, capacitance, resistance,
conductance, and dissipation factor. The desired function is selected by pushbutton. The range switch
is normally set to the automatic (AUTO) position for convenience.However, when a number of
similar measurements are to be made, it is faster to use the appropriate range instead of the automatic
range selection. The numerical value of the measurement is indicated on the 3 1/2-digit display, and
the multiplier and measured quantity are identified by LED indicating lamps.
2.9 SPECIFICATIONS OF DIGITAL LCR METER:
1. Display: 3-1/2 display(L/C/R-maximum 9999 display)
2. Parameters: D/Q- maximum 999 display
3. Tolerance mode: 1%, 5%, 10%
4. Measurement: 120Hz &41kHz
5. Measurement rate: 1 measurement per second, normal
6. Calibration: calibrates the meter internal parameters as well as external connector residues
7. Resistance: Range 10Ω to 10MΩ(±0.6%+5 dgts)
8. Inductance: Range 1mH to 1000H (±0.3%+5dgts)
9. Capacitance: Range 10nF to 10mf (±2.5%+5dgts)
10. Operation temp: 0 0c to 40 0c
11. Power: 6W max
12. Power requirement: AC 230/50 Hz
13. Dimensions: 211(L)x260(W)x71(H)
14. Weight: 1.64 Kg
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 19
2. DIGITAL INSTRUMENTS
UNIT-1
1.
List the advantages and disadvantages of PMMC meter (apr-10,nov-09)
2. Explain about rectifier type ammeter(apr-10)
3. Explain the FET-input electronic voltmeter(apr-10,nov-09,apr-09,nov-07)
4. Explain the capacitance measurement using the Schering bridge(apr-10)
5. Explain the series type ohm meter(apr-10,apr-09)
6. Explain high voltage and current probes(apr-10,nov-08)
7. Applications of wheat stone bridge(nov-09)
8. Explain loading effect of a voltmeter(nov-09,apr-09,nov-08,nov-07)
9. Explain how to extend the range of DC ammeter(nov-09,apr-09)
10. Explain the inductance measurement using Maxwell’s bridge(nov-09,nov-08)
11. Explain the working of basic differential voltmeter(apr-09,nov-08)
12. Explain the resistance measurement using wheat-stone bridge(apr-09,nov-07)
13. Principle of extending the range of DC voltmeter(nov-08)
14. Explain the working of shunt type ohmmeter (nov-08)
15. Explain the working principal of PMMC(nov-07)
UNIT-2
1. List the specifications of digital multimeter (apr-10,apr-09)
2. Explain the accuracy of a frequency meter(apr-10)
3. Explain the working of successive approximation type DVM(apr-10,nov-09)
4. List the specifications of digital frequency meter(apr-10,nov-09,apr-09,nov-07)
5. Explain the working of digital frequency meter(apr-10apr-09)
6. Mention the advantages of digital instruments over analog instruments(nov-09,nov-07)
7. List the specifications of digital LCR meter(nov-09,apr-09,nov-08)
8. List the specifications of DVM(nov-09,nov-08)
9. Explain the ramp type DVM(apr-09,nov-07)
10. Explain the working of digital multimeter(nov-08,nov-07)
11. Explain the dual slope integrating type DVM(nov-08)
A.A.N.M & V.V.R.S.R POLYTECHNIC
Page 20