The sensitivity of a digital voltmeter is
Answer: A
For a four and a half digit instrument with a 200 mV lowest range that is ten microvolts. It follows from the resolution and the lowest range together, and the ohms per volt figure belongs to analogue instruments rather than to digital ones.
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Normal mode rejection in a digital voltmeter refers to the rejection of interference appearing
Answer: B
Normal mode noise rides on the wanted signal itself, and an integrating instrument rejects it by averaging over a whole number of its cycles. Common mode noise, by contrast, appears on both terminals together and is rejected by the differential input and the guard.
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The integrating period of a digital voltmeter is made 20 ms in India in order to reject
Answer: C
Twenty milliseconds is exactly one period of fifty hertz, and the average of a sinusoid over its own period is zero. In a sixty hertz country the period is 16.67 ms, and instruments intended for both allow the line frequency to be selected.
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Common mode rejection ratio of a digital voltmeter expresses how well it ignores
Answer: D
A large voltage may exist between the low input terminal and earth, for instance when measuring across a shunt in a live circuit. A good differential input responds only to the difference between its terminals and ignores what they share.
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The guard terminal of a digital voltmeter is connected to
Answer: A
The guard is an internal shield surrounding the input circuit, and connecting it to the source's own low point makes the common mode current flow in the guard rather than through the measuring circuit. Connected wrongly it can make matters worse rather than better.
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Auto-zeroing in a digital voltmeter is the process of
Answer: B
A phase of each measuring cycle is spent with the input shorted, and the offset then found is subtracted from the reading proper. This removes amplifier drift and thermal offsets continuously, which is why a modern instrument needs no zero control at all.
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Auto-ranging in a digital voltmeter selects the range in which
Answer: C
Because the fixed count term of the accuracy specification is a smaller fraction of a larger reading, the most accurate range is the most sensitive one that does not overrange. The instrument steps down until it does overrange and then steps back up.
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Auto-polarity in a digital voltmeter means that it
Answer: D
Correct answer: displays a minus sign rather than requiring the leads to be reversed
Auto-polarity means the instrument senses the sign of the input and shows it, so the operator need not reverse the leads to read a negative voltage - a convenience an analogue meter cannot offer, since its pointer would simply drive backwards against the stop. Nothing is reversed internally and nothing is rejected.
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A digital multimeter measures alternating voltage by
Answer: A
The converter works on a steady voltage, so the alternating input must first be turned into one. How faithfully that conversion represents the root mean square value is what distinguishes an average responding instrument from a true rms one.
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An average responding digital multimeter is calibrated to read the root mean square value of
Answer: B
The rectified mean of a sinusoid is 0.9 of its rms value, and the scale factor built in assumes exactly that. On a distorted waveform the assumption fails, and the error can be tens of per cent on a peaky current such as that drawn by a switched-mode supply.
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