The seven-segment display of a digital voltmeter is driven from the counter through
Answer: C
Each decimal digit of the count is presented to a decoder whose seven outputs light the appropriate segments. Multiplexing lets one decoder serve every digit position in turn, which is why the display is strobed rather than continuously lit.
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A digital voltmeter used to measure a thermocouple output of a few millivolts requires above all
Answer: D
A thermocouple gives tens of microvolts per degree from a source of a few ohms, so input impedance hardly matters while drift and hum rejection matter enormously. Auto-zeroing and a twenty millisecond integration are exactly what such a measurement needs.
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Two digital voltmeters of different makes read 5.001 V and 4.998 V on the same source. The proper conclusion is that
Answer: A
Three millivolts on five volts is 0.06 per cent, which is comfortably within the specification of ordinary bench instruments. Deciding which is nearer the truth requires a standard traceable to a national laboratory, not a comparison between the two.
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The specification most relevant when measuring across a shunt in the negative rail of a supply floating at 400 V is
Answer: B
Both input terminals sit near four hundred volts with respect to earth while the wanted signal is a few millivolts between them. Only a well guarded differential input with a high common mode rejection ratio can make that measurement at all.
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An operator measures a 1.5 V cell and obtains 1.48 V, then repeats it with the leads reversed and obtains negative 1.48 V. This shows that
Answer: C
Equal magnitudes in both directions rule out an offset, which would add in one direction and subtract in the other. The reading of 1.48 V is simply the terminal voltage of a cell no longer quite fresh, which is what a voltmeter should report.
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The chief reason a modern digital multimeter has no zero adjustment on its voltage ranges is that
Answer: D
The instrument measures its own offset with the input shorted in one phase of each cycle and subtracts it, so drift is removed continuously and there is nothing left for the operator to set. An analogue instrument's pointer has no such facility and needs a mechanical zero.
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The lowest voltage range of a four and a half digit multimeter is 200 mV, so its sensitivity is
Answer: A
The count runs to 20000, so 200 mV divided by twenty thousand is ten microvolts. That is the smallest change the instrument can show at all, and whether it can be trusted depends on the drift and noise figures rather than on the display.
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Comparing an integrating digital voltmeter with an oscilloscope for measuring a steady voltage, the voltmeter is better because it
Answer: B
An oscilloscope resolves perhaps a fortieth of a division, which is two or three per cent of the screen, while a voltmeter resolves parts in twenty thousand and averages the noise besides. The oscilloscope's advantage lies in showing shape, not magnitude.
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A digital voltmeter is specified with a normal mode rejection of 60 dB at 50 Hz. A hum of 1 V peak on the input would produce an error of about
Answer: C
Correct answer: 1 mV
60 decibels is a voltage ratio of \[ 10^{60/20} = 1000, \] so a 1 V interfering signal appears as \( 1/1000 = 1\ \mathrm{mV} \) of error. Normal mode rejection describes interference riding on the measured signal itself, and it is achieved chiefly by the integration over whole mains cycles - which is why an integrating voltmeter is preferred where hum is unavoidable.
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The reason an integrating digital voltmeter is described as taking a mean rather than a sample is that its converter
Answer: D
The integrator's output at the end of the charging phase is proportional to the time integral of the input, which divided by the fixed period is the mean. Nothing is sampled at all, and that is why periodic interference cancels so completely.
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