Keysight 34972A - Sources of Error in AC Voltage Measurements

Tutorial 7
Keysight 34970A/34972A User’s Guide 297
Sources of Error in AC Voltage Measurements
Many of the errors associated with DC voltage measurements also apply to AC
voltage measurements. Additional errors unique to AC voltage measurements are
described in this section.
Crest Factor Errors (non-sinusoidal inputs)
A common misconception is that “since the internal DMM is true RMS, its
sinewave accuracy specifications apply to all waveforms.” Actually, the shape of
the input signal can dramatically affect measurement accuracy. A common way to
describe signal waveshapes is crest factor. Crest factor is the ratio of the peak
value to the RMS value of a waveform.
For a pulse train, for example, the crest factor is approximately equal to the square
root of the inverse of the duty cycle as shown in the table on page 295. In general,
the greater the crest factor the greater the energy contained in higher frequency
harmonics. All multimeters exhibit measurement errors that are crest factor
dependent. Crest factor errors are shown in the specifications in Chapter 8. Note
that the crest factor errors do not apply for input signals below 100 Hz when using
the slow AC filter.
You can estimate the measurement error due to signal crest factor as shown
below:
Total Error = Error
sine
+ Error
crest factor
+ Error
bandwidth
Where:
Error
sine
= DMM’s sinewave accuracy (see Chapter 8, "Specifications")
Error
crest factor
= DMM’s crest factor (see Chapter 8, "Specifications")
Error
bandwidth
= Estimated bandwidth error as shown below:
Error
bandwidth
=
Where:
C.F. = Signal crest factor (see the table on page 295)
F = Fundamental input signal frequency
BW = DMM’s -3 dB bandwidth (1 MHz for the 34970A/34972A)
C.F.
2
– F×
4π BW×
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