System Switch/Multimeter User's Manual Section 5:
Basic Digital Multimeter (DMM) Operation
-900-01 Rev. A / August 2007 5-
The reason to consider crest factor in accuracy of RMS measurements is because the meter
has a limited bandwidth. Theoretically, a sine wave can be measured with a finite bandwidth
because all of its energy is contained in a single frequency. Most other common waveforms
have a number of spectral components requiring an almost infinite bandwidth above the
fundamental frequency to measure the signal exactly. Because the amount of energy contained
in the harmonics becomes smaller with increasing frequency, very accurate measurements can
be made with a limited bandwidth meter, as long as enough spectral components are captured
to produce an acceptable error.
Crest factor is a relative measurement of the harmonic content of a particular waveform and
reflects the accuracy of the measurement. For a rectangular pulse train, the higher the crest
factor, the higher the harmonic content of the waveform. This is not always true when making
spectral comparisons between different types of waveforms. A sine wave, for example, has a
crest factor of 1.414, and a square wave has a crest factor of 1. The sine wave has a single
spectral component and the square wave has components at all odd harmonics of the
fundamental.
The Series 3700 RMS AC volts and AC amps accuracies are specified for sine waves of
different frequency ranges.
Additional error uncertainties are also specified for non-sinusoidal waveforms of specific crest
factors and frequencies. The Series 3700 has capabilities of measuring AC waveforms of crest
factors up to 5.
Speed, accuracy, and settling times for AC current and voltage
The Series 3700 is an AC-coupled RMS meter. For an AC waveform with a DC content, the DC
component is removed before the RMS is calculated. The RMS converter rectifies the AC
waveform and creates a proportional DC level. The amount of the DC level is dependent on
input level, frequency, and dmm.detectorbandwidth setting.
When detectorbandwidth is between 3 and 30
For dmm.detectorbandwidth=3 or 30, the A/D uses a multi-sampling algorithm to measure the
DC and AC output of the RMS converter. These settings have been optimized for inputs from 3-
300kHz, dmm.detectorbandwidth=3 (SLOW) and 30Hz to 300kHz for
dmm.detectorbandwidth=30 (MED). The number of samples taken and the intervals vary with
the settings as follows:
Also, for varying input levels, a 330msec delay is required to properly settle the RMS converter's
filter capacitor.
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