Digital Voltmeters Questions and Answers

Q: A dual slope digital voltmeter has a gate time of 400 ms reflecting the measurement cycle. The frequency bandwidth of this instrument is :

The bandwidth is 1.25 Hz.A dual slope voltmeter integrates the input over a fixed time and so behaves like an averaging filter. For an integration (measurement) time T, the effective bandwidth of such an averaging process is taken as \(1/2T\):\[ \begin{aligned} B &= \frac{1}{2T} = \frac{1}{2 \times 0.4\ \text{s}} \\ &= 1.25\ \text{Hz} \end{aligned} \]The option 2.5 Hz is \(1/T\), the measurement rate, not the bandwidth. The values 0.8 Hz and 0.6 Hz do not follow from the gate time.

Q: Match List-I with List-II :List-I : Number on DisplayList-II : ADC(a) 1072(i) 8-bit(b) 893(ii) 10-bit(c) 132(iii) 12-bit(d) 59705(iv) 16-bit

The correct matching is a-iii, b-ii, c-i, d-iv.An n-bit ADC can show the codes 0 to 2n - 1, so a display reading needs the smallest ADC whose top code is not exceeded:\[ \text{8-bit: }0\text{ to }255 \]\[ \text{10-bit: }0\text{ to }1023 \]\[ \text{12-bit: }0\text{ to }4095 \]\[ \text{16-bit: }0\text{ to }65535 \]132 is at most 255, so an 8-bit ADC is enough: (c) with (i). 893 is above 255 and at most 1023, so 10-bit: (b) with (ii). 1072 is above 1023 and at most 4095, so 12-bit: (a) with (iii). 59705 is above 4095 and at most 65535, so 16-bit: (d) with (iv).The other codes break at least one of these ranges, for example putting 893 on an 8-bit ADC, which cannot show a number above 255.

Q: Which of the following would need a 4½ digit digital multimeter ?

The reading that needs a 4½ digit meter is 1.4856 mV.A 4½ digit display has four full digits (0-9) and a leading half digit that can only be 0 or 1, so its maximum count is 19999. The reading 1.4856 has five digits with a leading 1, which fits exactly.1.999 mA and -1.999 µV have four digits with a leading 1, which a 3½ digit meter (maximum count 1999) can already show, and so can 1000 V. Only 1.4856 mV requires the extra digit.

Q: The resolution of a 3½ digit DMM on 1 V and 10 V is :

The resolutions are 0.001 V and 0.01 V.A 3½ digit meter has a maximum count of 1999, so its resolution is one count in the last digit. On the 1 V range the display reads up to 1.999 V, so the smallest step is 0.001 V. On the 10 V range it reads up to 19.99 V, so the smallest step is 0.01 V.\[ \begin{aligned} \frac{1\ \text{V}}{1000} &= 0.001\ \text{V} \\ \frac{10\ \text{V}}{1000} &= 0.01\ \text{V} \end{aligned} \]The other options are ten or a hundred times too coarse on one or both ranges.

A dual slope digital voltmeter has a gate time of 400 ms reflecting the measurement cycle. The frequency bandwidth of this instrument is :

A.2·5 Hz
B.1·25 Hz
C.0·8 Hz
D.0·6 Hz

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

The bandwidth is 1.25 Hz.

A dual slope voltmeter integrates the input over a fixed time and so behaves like an averaging filter. For an integration (measurement) time T, the effective bandwidth of such an averaging process is taken as \(1/2T\):

\[ \begin{aligned} B &= \frac{1}{2T} = \frac{1}{2 \times 0.4\ \text{s}} \\ &= 1.25\ \text{Hz} \end{aligned} \]

The option 2.5 Hz is \(1/T\), the measurement rate, not the bandwidth. The values 0.8 Hz and 0.6 Hz do not follow from the gate time.

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Match List-I with List-II :

List-I : Number on Display	List-II : ADC
(a) 1072	(i) 8-bit
(b) 893	(ii) 10-bit
(c) 132	(iii) 12-bit
(d) 59705	(iv) 16-bit

A.a-iii, b-i, c-ii, d-iv
B.a-iii, b-ii, c-i, d-iv
C.a-iv, b-i, c-ii, d-iii
D.a-i, b-ii, c-iii, d-iv

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The correct matching is a-iii, b-ii, c-i, d-iv.

An n-bit ADC can show the codes 0 to 2ⁿ - 1, so a display reading needs the smallest ADC whose top code is not exceeded:

\[ \text{8-bit: }0\text{ to }255 \]

\[ \text{10-bit: }0\text{ to }1023 \]

\[ \text{12-bit: }0\text{ to }4095 \]

\[ \text{16-bit: }0\text{ to }65535 \]

132 is at most 255, so an 8-bit ADC is enough: (c) with (i). 893 is above 255 and at most 1023, so 10-bit: (b) with (ii). 1072 is above 1023 and at most 4095, so 12-bit: (a) with (iii). 59705 is above 4095 and at most 65535, so 16-bit: (d) with (iv).

The other codes break at least one of these ranges, for example putting 893 on an 8-bit ADC, which cannot show a number above 255.

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Which of the following would need a 4½ digit digital multimeter ?

A.1.999 mA
B.-1.999 µV
C.1000 V
D.1.4856 mV

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The reading that needs a 4½ digit meter is 1.4856 mV.

A 4½ digit display has four full digits (0-9) and a leading half digit that can only be 0 or 1, so its maximum count is 19999. The reading 1.4856 has five digits with a leading 1, which fits exactly.

1.999 mA and -1.999 µV have four digits with a leading 1, which a 3½ digit meter (maximum count 1999) can already show, and so can 1000 V. Only 1.4856 mV requires the extra digit.

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The resolution of a 3½ digit DMM on 1 V and 10 V is :

A.0.1 V and 0.01 V
B.0.01 V and 0.1 V
C.0.001 V and 0.1 V
D.0.001 V and 0.01 V

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

The resolutions are 0.001 V and 0.01 V.

A 3½ digit meter has a maximum count of 1999, so its resolution is one count in the last digit. On the 1 V range the display reads up to 1.999 V, so the smallest step is 0.001 V. On the 10 V range it reads up to 19.99 V, so the smallest step is 0.01 V.

\[ \begin{aligned} \frac{1\ \text{V}}{1000} &= 0.001\ \text{V} \\ \frac{10\ \text{V}}{1000} &= 0.01\ \text{V} \end{aligned} \]

The other options are ten or a hundred times too coarse on one or both ranges.

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The most suitable instrument for the measurement of voltage is

A.DVM
B.Analog voltmeter
C.CRO
D.DMM

Exam Year(s)

UGC NET - 2012,

Answer: C

The key takes the CRO as the most suitable instrument for measuring voltage (option C). It shows the actual waveform, so it measures the peak, peak-to-peak and instantaneous values of any waveform, including non-sinusoidal and fast signals, and it has a high input impedance.

An analog voltmeter (B) and a DMM (D) are calibrated for steady or sinusoidal values and respond slowly to a changing signal, and a DVM (A) gives a number without showing the waveform. The question has no stated condition, so the answer rests on the key.

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In a DVM, the accuracy of the measurement depends on

I. No. of A/D conversions in a second.
II. The resolution of A/D converter.
III. A statistical processing of samples and a huge memory.
IV. Speed of the processor should be high.

A.I, II and III are to be fulfilled
B.II, III and IV are to be fulfilled
C.I, III and IV are to be fulfilled
D.I, II and IV are to be fulfilled

Exam Year(s)

UGC NET - 2012,

Answer: B

The key takes statements II, III and IV as the conditions to be fulfilled, option (B). The accuracy of a digital voltmeter depends on the resolution of the A/D converter, statement II, because the smallest step sets the quantisation error. It also depends on statistical processing of the samples with enough memory, statement III, which averages out noise, and on a fast processor, statement IV, so that the processing keeps up. Statement I, the number of conversions in a second, is not counted by the key as a determinant of accuracy, which it treats as a matter of speed. Options (A), (C) and (D) include I. This item follows the key.

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Direction for [ Question No: 7 To 7 ] :

Assertion – Reason type questions :
The following items consist of two statements, one labelled the ‘Assertion (A)’ and the other labelled the ‘Reason (R)’. You are to examine these two statements and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer sheet accordingly.

Codes :
(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
(B) Both (A) and (R) are true but (R) is not the correct explanation of (A).
(C) (A) is true and (R) is false.
(D) (A) is false and (R) is true.

Assertion (A) : Dual slope A/D converter is the most preferred A/D conversion technique employed in most digital multimeters.

Reason (R) : Dual slope A/D converter provides high accuracy and also suppresses the HUM effect on the input signal.

A.Both (A) and (R) are true and (R) is the correct explanation of (A).
B.Both (A) and (R) are true but (R) is not the correct explanation of (A).
C.(A) is true and (R) is false.
D.(A) is false and (R) is true.

Exam Year(s)

UGC NET - 2012,

Answer: A

Both statements are true and the reason explains the assertion. The dual slope converter integrates the unknown input for a fixed time and then integrates a reference of opposite polarity until the capacitor is discharged, so the result depends on the ratio of two times and not on the accuracy of the clock or of the integrating components. Because the input is averaged over a fixed period, hum picked up at the mains frequency is suppressed when that period is a multiple of the mains cycle. This accuracy and noise rejection are the reasons why the dual slope converter is preferred in digital multimeters. The answer is that both are true and (R) is the correct explanation of (A).

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The essential operation performed by every digital voltmeter is

A.analogue to digital conversion
B.amplification of the input to a level the display can accept
C.rectification of the input so that a steady quantity may be measured
D.comparison of the input with a standard cell of known electromotive force

Answer: A

Whatever the internal method, the instrument must turn a continuous voltage into a number. The differences between the types lie entirely in how that conversion is arranged and in the accuracy, speed and noise immunity each arrangement gives.

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In a ramp type digital voltmeter the quantity actually counted is

A.the number of zero crossings of the input during a fixed interval
B.the time for a linear ramp to pass the input voltage
C.the number of clock pulses needed to charge a capacitor to the input voltage
D.the number of standard steps required to equal the input voltage

Answer: B

A ramp starts above the input and falls; a gate opens when it crosses the input and closes when it crosses zero, and the clock pulses passed in that interval are proportional to the input. It is simple and quick but depends on the linearity of the ramp and the stability of the clock.

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10.

The chief weakness of the ramp type digital voltmeter is that

A.it takes far longer to make a reading than any other type
B.it requires a standard cell to be maintained within the instrument
C.its accuracy depends on the linearity of the ramp
D.it cannot measure a voltage of negative polarity at all

Answer: C

Any curvature or drift in the ramp generator appears directly as an error in the reading, and the clock frequency must be equally stable. The dual slope method removes both dependencies at once, which is why it displaced the plain ramp.

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Digital Voltmeters Questions and Answers

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