Digital Voltmeters Questions and Answers

Answer: A

Both statements are true and the reason explains the assertion. The dual slope converter integrates the unknown input for a fixed time and then integrates a reference of opposite polarity until the capacitor is discharged, so the result depends on the ratio of two times and not on the accuracy of the clock or of the integrating components. Because the input is averaged over a fixed period, hum picked up at the mains frequency is suppressed when that period is a multiple of the mains cycle. This accuracy and noise rejection are the reasons why the dual slope converter is preferred in digital multimeters. The answer is that both are true and (R) is the correct explanation of (A).

Answer: A

Whatever the internal method, the instrument must turn a continuous voltage into a number. The differences between the types lie entirely in how that conversion is arranged and in the accuracy, speed and noise immunity each arrangement gives.

Answer: B

A ramp starts above the input and falls; a gate opens when it crosses the input and closes when it crosses zero, and the clock pulses passed in that interval are proportional to the input. It is simple and quick but depends on the linearity of the ramp and the stability of the clock.

Answer: C

Any curvature or drift in the ramp generator appears directly as an error in the reading, and the clock frequency must be equally stable. The dual slope method removes both dependencies at once, which is why it displaced the plain ramp.

Answer: D

A counter drives a digital to analogue converter whose output climbs one step at a time until it exceeds the input, and the count then standing is displayed. The reading depends on the accuracy of the converter rather than on the linearity of an analogue ramp.

Answer: A

The fixed charging time gives a ramp height proportional to the mean input; the reference then discharges it at a known rate and the time to reach zero is measured. Since the same capacitor and clock serve both phases, their values cancel out of the result.

Answer: B

The ratio of the discharge time to the charge time is the ratio of the input to the reference, and the capacitance and the clock rate cancel in forming that ratio. This is the great structural merit of the method and the reason it dominates bench instruments.

Answer: C

The average of a sinusoid over an exact number of its own periods is zero, so an integrating period of 20 ms, or a multiple of it, removes fifty hertz interference completely. The integrating period is switched to 16.67 ms in a sixty hertz country.

Answer: D

The most significant bit is tried, kept if the trial voltage is still below the input and discarded otherwise, and so on down the word. An n-bit conversion therefore takes exactly n comparisons, which makes it both fast and of predictable duration.

Answer: A

One comparison settles each bit, so twelve bits take twelve comparisons whatever the input. A counting converter of the same resolution might need up to 4096 clock periods, which is why successive approximation is used where speed matters.