Review Question:

1. State the advantages of a DVM over an analog meter.

• Digital display of the output eliminates human reading errors.

• Readings are accurate and fast compared to analog meters.

• Digital Voltmeter is more stable and reliable.

• Smaller in size and cost-effective.

• DVM can measure both AC and DC voltages.

2. How are DVMs classified?

• RAMP TECHNIQUE

• DUAL SLOPE INTEGRATING TYPE DVM

• Integrating type DVM

3. State the operating and performance characteristics of a

digital voltmeter
• Input range from + 1.000 V to + 1000 V with automatic range selection and

overload indication

• Absolute accuracy as high as ±0.005% of the reading

• Resolution 1 part in million (1 Jl,Vreading can be read or measured on 1 V

range)

• Input resistance typically 10 MQ, input capacitance 40 pF

• Calibration internally from stabilised reference sources, independent of

measuring circuit
 • Output in BCD form, for print output and further digital processing.

Optional features may include additional circuitry to measure current,

ohms and voltage ratio.

4. State distinguished features of digital instruments as

compared to analog instruments.
The DVM displays ac and de voltages as discrete numbers, rather than as a

pointer on a continuous scale as in an analog voltmeter. A numerical readout is

advantageous because it reduces human error, eliminates parallax error,

increases reading speed and often provides output in digital form suitable for

further processing and recording

5. Explain the operating principle of a ramp type DVM.

The operating principle is to measure the time that a linear ramp takes to change

the input level to the ground level, or vice-versa. This time period is measured with

an electronic time-interval counter and the count is displayed as a number of

digits on an indicating tube or display

Or
based on the measurement of the time which it takes for a linear ramp voltage to

increase from zero level to the input voltage level, or to decrease from the level of

the input signal to zero

6. Describe with a diagram the operation of ramp type DVM.
State limitations of a ramp type DVM and how it is overcome.
7. Explain with the help of diagram the working principle of
dual slope type DVM.
Principle of Dual Slope Type DVM As illustrated in Fig. 5.3, the input voltage 'e/ is
integrated, with the slope of the integrator output proportional to the test input

voltage. After a fixed time, equal to t., the input voltage is disconnected and the

integrator input is connected to a negative voltage -er. The integrator output will

have a negative slope which is constant and proportional to the magnitude of the

input voltage.
8. Describe with a diagram the operation of a dual slope type
DVM.

9. State the advantages of a dual slope DVM over a ramp type

DVM

The dual slope technique has excellent noise rejection because noise and
superimposed ac are averaged out in the process of integration. The speed and
accuracy are readily varied according to specific requirements; also an accuracy of ±
0.05% in 100 ms is available.
10. Explain the operating principle of Voltage to frequency
type DVM.

A constant input voltage is integrated and the slope of the output ramp is
proportional to the input voltage. When the output reaches a certain value, it is
discharged to 0 and another cycle begins. The frequency of the output waveform is
propor-tional to the input voltage.
11. Describe with the help of a block diagram the operation of
integrating type digital voltmeter

The input voltage produces a charging current, e/R I,that charges the capacitor 'C' to
the reference voltage e,.. When e, is reached, the comparator changes state, so as to
trigger the precision pulse generator. The pulse generator produces a pulse of
precision charge content that rapidly discharges the capacitor. The rate of charging
and discharging produces a signal frequency that is directly proportional to ei.
Multiple Choice Questions:
1. Measurement in a ramp type DVM is performed during the

A. negative slope

B. positive slope

C. both of the slope

D. none of the above

Answer: (A)

2. Measurement by dual slope DVM is performed during

A. rising slope

B. falling slope

C. rising and falling slope

D. none of the slope

Answer: (C)
3. The principle of voltage to time conversion is used in

A. dual slope type DVM

B. successive approximation type DVM

C. integrating type DVM

D. none of the above

Answer: (A)

4. Dual slope operates on the principle of

A. voltage to time conversion

B. voltage to frequency conversion

C. frequency to voltage conversion

D. voltage to current conversion

Answer: (A)
Practice Problems:
1. The lowest range on a 4-1/2 digit multimeter is 10m V full
scale. Determine the sensitivity of the meter?
Solution:
Number of full digits is 4. Therefore,
1
Resolution=10𝑁

Where, n = 4
1
Resolution=104 = 0.0001

Sensitivity S = (ft)min x R

where (fs)min = lowest full scale of the meter

R = resolution expressed as decimal

Therefore, Sensitivity S = (fs)min x R

Where R = 0.0001 . (fs)min = 10m V

Sensitivity S = 10m * 0.0001 = 0.000001𝑉

𝛺
2. The lowest range on a 3-1/2 digit multimeter is 10m V full
scale. Determine the sensitivity of the meter.
Solution:
Number of full digits is 3. Therefore,
1
Resolution=10𝑁

Where, n = 3
1
Resolution=103 = 0.001

Sensitivity S = (ft)min x R

where (fs)min = lowest full scale of the meter

R = resolution expressed as decimal

Therefore, Sensitivity S = (fs)min x R

Where R = 0.001 . (fs)min = 10m V

Sensitivity S = 10m * 0.001 = 0.00001𝑉

𝛺

3. A 3-1/2 digit DVM is used for measuring voltage .Determine

the resolution. How would a voltage of 14.42 be displayed on
10 V range and 100 V range
Solution:
Number of full digits is 3. Therefore,
1
Resolution=10𝑁

Where, n = 3
1
Resolution=103 = 0.001

For full scale range reading of 1 V, the resolution is 10 x 0.001 = 0.01 V.

therefore 14.42 would be displayed as 14.42 v

For full scale reading of 10 V range, the resolution is 100V x 0.001 = 0.1 V.
therefore 14.42 would be displayed as 14.4 v

4. A 3-1/2 digit DVM has an accuracy of +0.5% of reading + 1

digit. Determine the possible error in V, when the instrument
is reading 5 V in 10 V range
Solution:
Voltmeter reading = 5

full scale reading = 10

Number of full digits = 3

1
Resolution=10𝑁

Where, n = 3
1
Resolution= X 10= 0.01 V
103

The voltage corresponding to one digit = 0.01 V

error in V = +0.5% of displaying 5v reading

= 0.005 * 5 +0.01 = 0.035 V