Digital Voltmeters Questions and Answers

Answer: C

Integration averages the noise over the measuring period, and choosing that period as a whole number of mains cycles cancels supply frequency interference outright. Every sampling method takes the input as it finds it, noise included.

Answer: D

Correct answer: the successive approximation type

Successive approximation resolves one bit per clock period, so an n-bit conversion takes n cycles and completes in microseconds. The integrating types are far slower by design: a dual slope converter integrates for a fixed period, usually a whole number of mains cycles, precisely in order to reject interference - and that averaging is bought with time.

Answer: A

The half digit is the most significant and can display only a blank or a one, so the count runs from 0000 to 1999. Overrange beyond that requires the next range to be selected, which is what auto-ranging does automatically.

Answer: B

Full scale corresponds to a count of 2000, so the smallest distinguishable step is a two-thousandth of the range, or 0.05 per cent. Resolution is a property of the display and the converter, and is quite separate from accuracy.

Answer: C

Correct answer: 1 mV

Four and a half digits means four full digits and a leading half digit, giving a full scale of 19999 counts. On the 20 V range each count is therefore \[ \frac{20\ \mathrm{V}}{20{,}000} = 1\ \mathrm{mV}. \] Resolution is the value of one count and follows from the digit count and the range together; it is not the same thing as accuracy, which the specification states separately.

Answer: D

An instrument may resolve a microvolt and yet be wrong by a millivolt. Resolution says how finely the reading is divided; accuracy says how far the reading may be from the truth, and it is always the coarser of the two.

Answer: A

The first term is proportional to the reading and comes from gain error; the second is a fixed number of counts and comes from offset and quantisation. Because the second term is fixed, the percentage error grows sharply as the reading falls toward the bottom of a range.

Answer: B

Three counts on a reading of 1900 is a sixth of one per cent; the same three counts on a reading of 100 is three per cent. The remedy is to select a lower range so that the reading sits high in it, which is exactly what auto-ranging achieves.

Answer: C

A source of appreciable internal resistance would be loaded by a low impedance instrument and its voltage would fall, so the reading would be low. Ten megohms loads all but the most delicate sources negligibly, and the very best instruments offer more still.

Answer: D

The instrument puts ten megohms across ten megohms, giving five, so the divider ratio changes from a half to a third. The reading is therefore two thirds of the true value. High resistance circuits need an electrometer input or a much higher impedance instrument.