Digital Voltmeters Questions and Answers

Answer: B

Four bits per decimal digit is a convenient form for logging equipment to read, and it was the standard interface before serial buses. A chart recorder would need an analogue output instead, which some instruments also provide.

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.

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.

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.

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.

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.

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.

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.

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.

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.