The quantisation error of a converter is at most
Answer: A
Rounding to the nearest level puts the true value at most half a step away from the number reported. That half bit is irreducible and independent of the quality of the reference or the amplifier, so it is the ultimate limit of a converter of that resolution.
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A voltmeter of 10 M\( \Omega \) input impedance measures a source of 1 M\( \Omega \) internal resistance. The reading is low by about
Answer: B
The instrument and the source form a divider, so the reading is \( 10/11 = 0.909 \) of the true value, low by about nine per cent. A source resistance of a tenth of the input impedance is therefore already a serious matter.
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A three and a half digit meter displays 1999 and then blanks all but the leading one when the input is raised. This indicates
Answer: C
Beyond a count of 1999 the display cannot represent the number, so an overrange indication is given and a higher range must be selected. Auto-ranging instruments do this themselves and the indication is seen only at the top of the highest range.
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An average responding multimeter is used on a square wave of 10 V peak. It will read about
Answer: D
The rectified mean of the square wave is its peak, ten volts, and the instrument multiplies by 1.11 on the assumption of a sinusoid. The true rms value is ten volts, so the reading is eleven per cent high, and a true rms instrument would report it correctly.
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A true rms multimeter with a crest factor limit of 3 is used on a waveform whose peak is five times its rms value. The reading will be
Answer: A
The input amplifier saturates on the peaks, so part of the waveform never reaches the computing circuit and the squared average comes out too small. There is no warning indication, which makes it a genuine hazard when measuring rectifier currents.
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True or false: a digital voltmeter with more digits is necessarily more accurate.
Answer: B
More digits give finer resolution, but the reference, the amplifier and the attenuator determine how close the reading is to the truth. A five digit instrument with a drifting reference can be less accurate than a well made three and a half digit one.
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True or false: the dual slope method makes the reading independent of the clock frequency.
Answer: C
The reading is a ratio of two times, both measured with the same clock, so the clock rate cancels. The same is true of the integrating capacitor, which is why the method needs neither a precision capacitor nor a crystal oscillator to be accurate.
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True or false: an integrating digital voltmeter set for fifty hertz rejects sixty hertz interference equally well.
Answer: D
Twenty milliseconds is 1.2 periods of sixty hertz, so the average over it is not zero and the rejection is poor. Instruments intended for both regions offer a line frequency selection that changes the integrating period accordingly.
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True or false: auto-zeroing removes the need for the operator to short the leads before a reading.
Answer: A
A phase of every measuring cycle is spent measuring the instrument's own offset with the input shorted internally, and that offset is subtracted. Drift is therefore corrected continuously and no manual zeroing is possible or necessary.
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Assertion: the integrating period of a digital voltmeter is made a whole number of mains periods. Reason: the average of a sinusoid over its own period is zero.
Answer: B
Mains hum superimposed on the input averages exactly to nothing over a whole number of its own cycles, so the integrator sees only the wanted signal. This is the entire reason for the twenty millisecond period and the reason given is the correct one.
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