# An integrating digital voltmeters measures

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UJVNL AE EE 2016 Official Paper
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1. True average value
2. rms value
3. Peak value
4. None of the above

## Answer (Detailed Solution Below)

Option 1 : True average value

## Detailed Solution

Concept:

Integrating type digital voltmeter (DVM):

It measures the voltage in digital domain. It is possible to do so using voltage to frequency conversion.

Reference voltage can be varied to change the duration of the pulse train (or we can say to change frequency of pulses)

This voltmeter employs an integration technique that uses a voltage to frequency conversion. The heart of this technique is the operational amplifier acting as an Integrator.

The output voltage of the integrator is
$$E_o=\frac{E_i}{RC}\times t$$

The basic block diagram of a typical integrating type of DVM is shown :

• Input voltage is fed to integrator and its output is sensed by level detector.
• Level detector discrete signal is matched with clock oscillator signal to generate train of pulses.
• Time base selector is used to select the duty ratio of pulses required.
• Combined we get duty cycle along with duration which is used to trigger the BCD counter circuit
• The count is read in LCD display.

Explanation:

The voltage measurement is proportional to frequency for a fixed reference value.

Since it reads a digital value from the converted actual analog voltage signal, the average value of the voltage signal is used for comparison.

Thus, DVM measures the true average value of the input voltage over a fixed measuring period.