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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A true rms digital multimeter obtains its reading by
Answer: C
Either an analogue computing circuit or a digital calculation from samples performs those three operations, so the reading is correct whatever the waveform. The specification will state a crest factor limit beyond which even a true rms instrument cannot cope.
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The crest factor limit quoted for a true rms multimeter is the largest permissible ratio of
Answer: D
A very peaky waveform drives the input circuits into clipping long before the rms value reaches full scale, so a limit of three or five is stated. Exceeding it gives a low reading with no warning, which is a trap when measuring the current of a rectifier load.
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Comparing a digital voltmeter with an analogue one, the digital instrument is inferior in
Answer: A
A moving pointer conveys direction and rate of change immediately, whereas a flickering set of digits conveys neither. This is why many digital instruments now include a bar graph alongside the numerals, to recover what the pointer gave.
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