103
Crest Factor Errors (non-sinusoidal inputs)
A common misconception is that "since an ac Multimeter is true RMS, its sine
wave accuracy specifications apply to all waveforms. “Actually, the shape of the
input signal can dramatically affect the measurement accuracy. A common way
to describe the signal wave shapes is “crest factor”. Crest factor is the ratio of the
peak value to RMS value of a waveform.
The greater the crest factor, the greater the energy contained in the high
frequency harmonics. All Multimeters have errors that are crest factor dependent.
(The crest factor errors do not apply for input signals below 100Hz.)
You can estimate the measurement error due to signal crest factor as shown
below:
Total Error
＝
Error (Sine wave) + Error (Crest factor) + Error (Bandwidth)
Error (Sine wave):
error for sine wave.
Error (Crest factor):
crest factor additional error.
Error (Bandwidth):
estimated bandwidth error as shown below:
Bandwidth error =
×100%
（
% reading
）
C
.
F
.
: signal crest factor
F
:
fundamental frequency of pulse
BW
:
effective bandwidth of the Multimeter
Example:
Calculate the approximate measurement error for a pulse train input with a crest
factor of 2 and a fundamental frequency of 20 kHz. For this example, assume
1- year accuracy specifications of the Multimeter: ± (0.05%× r
0.03%×range).
Total Error = (0.05%×0.03%×range) + (0.05%×range) +
(0.8%×reading)
=0.85%×r 0.08%×range
