Keysight DAQ970A Guia Del Usuario página 237

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.
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. 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:
Where:
Error = DMM's sinewave accuracy
sine
Error = DMM's crest factor
crest factor
Error =
bandwidth
Where:
C.F = Signal crest factor
F = Fundamental input signal frequency
BW = DMM's -3dB bandwidth (1 MHz for the DAQ970A/DAQ973A)
Example: Calculating measurement error
Calculate the approximate measurement error for a pulse train input with a crest factor of 3 and a fundamental
frequency of 20 kHz. The internal DMM is set to the 1 V range. For this example, use the 90-day accuracy
specifications of ± (0.05% of reading + 0.04% of range).
Error = = ±(0.05%+0.04%) = ±0.09%
sine
Error = 0.15%
crest factor
Error =
bandwidth
Total Error = 0.09% + 0.15 % + 1.4 % = 1.6 %
Keysight DAQ970A/DAQ973A User's Guide
4 Measurement Tutorials
237