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The dual slope digital voltmeter rejects mains frequency interference well because it

  • A.samples the input only at the instants when the mains voltage is zero
  • B.takes the mean of two readings a half cycle of the mains apart
  • C.integrates the input over a whole number of mains cycles
  • D.contains a notch filter tuned to the frequency of the supply

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.

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