A reading on a digital voltmeter flickers between two values while a nearby motor is running. The measure most likely to help is
Answer: D
A longer integration averages the interference over more of its cycles and so suppresses it further, at the cost of a slower reading rate. Twisted or screened leads with the guard properly connected would help further still.
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A digital voltmeter's reference is derived from
Answer: A
The reference is the one component whose value the reading depends on directly, so it is made as stable as possible against temperature and time. Its drift is the largest single contributor to the gain error term of the accuracy specification.
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The ohms function of a digital multimeter works by
Answer: B
A current source of known value drives the unknown and the resulting voltage drop is converted in the usual way, so the display can read directly in ohms. On the lowest ranges a four-terminal connection is offered to remove the lead resistance.
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The four-wire ohms connection offered by a good digital multimeter is provided so that
Answer: C
One pair of leads carries the current and the other senses the voltage at the specimen itself, so the drops in the current leads and their contacts do not enter. It is the same principle as the four-terminal resistor of low-resistance bridge work.
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The continuity buzzer of a digital multimeter responds when the measured resistance falls below
Answer: D
A threshold of perhaps thirty ohms lets a connection be traced by ear without watching the display, which is its whole convenience. It is a rough test and says nothing about whether a joint is good enough to carry current.
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The diode test function of a digital multimeter displays
Answer: A
A small current is passed and the resulting voltage displayed, so a silicon junction reads about 0.6 V and a light emitting diode rather more. The ohms function is unsuitable because a junction has no single resistance.
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A digital voltmeter reading of 0.000 with the leads open and 0.002 with them shorted on the two volt range indicates
Answer: B
Two counts on that range is two millivolts, and it appears only when the input is completed, so it is a thermal or contact offset in the leads rather than in the instrument. It matters when measuring millivolt signals and is dealt with by noting and subtracting it.
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The number of digits and the accuracy of a digital voltmeter are related in that a well designed instrument has
Answer: C
There is little point resolving a microvolt on an instrument accurate only to a millivolt, and equally little point in an accuracy the display cannot show. Makers generally provide a resolution a few times finer than the accuracy, so the last digit is informative though not guaranteed.
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Comparing the ramp and dual slope types on the same input containing mains hum, the ramp type
Answer: D
The ramp type takes the input as it stands at the instant of comparison, so a superimposed hum shifts the crossing point and the reading wanders. The dual slope type averages over a whole hum period and reports a steady value.
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A digital panel meter differs from a bench digital voltmeter chiefly in
Answer: A
It is built into equipment to display one quantity, so ranges, function switches and probes are unnecessary and the scaling is fixed by external resistors. The converter inside is usually the same integrating type as in a bench instrument.
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