Rigol DM858 - 8 Measurement Tutorial; True RMS AC Measurements; Crest Factor Errors (Non-Sinusoidal Inputs); Table 8.1 True RMS AC Measurement of Sine, Triangle and Square Waves

8
Measurement Tutorial
8.1 True RMS AC Measurements
The AC measurement of DM858 series has true RMS response. The power dissipated
in a resistor within a time is proportional to the square of the measured true RMS
voltage, independent of wave shape. This multimeter can accurately measure true
RMS voltage or current, as long as the wave shape contains negligible energy above
the multimeter's effective bandwidth. The effective AC voltage bandwidth of DM858
series is 8 kHz and the effective AC current bandwidth is 8 kHz.
The multimeter's AC voltage and AC current functions measure the "AC coupled" true
RMS value, that is to measure the RMS value of the AC component (DC component is
rejected) of the input signal. For sine waves, triangle waves, and square waves, the AC
and AC+DC values are equal since these waveforms do not contain a DC offset, as
shown in
Table 8.1 True RMS AC Measurement of Sine, Triangle and Square Waves .
Table 8.1 True RMS AC Measurement of Sine, Triangle and Square Waves
Waveform Crest Factor AC RMS
2
2
V
8.2 Crest Factor Errors (non-sinusoidal inputs)
A common misconception is that "since a multimeter can measure the true RMS of a
signal, its sine wave accuracy specifications apply to all waveforms."
Actually, the waveform shape of the input signal can affect the measurement
accuracy. Generally, "crest factor" is used to describe the signal wave shape. The crest
factor is the ratio of the peak value to RMS value of a waveform.
In general, the greater the crest factor is, the greater the energy contained in high
frequency harmonics will be. All of the multimeters have errors that are relevant to
the crest factor. Please note that the crest factor errors do not apply to input signals
below 100 Hz.
You can estimate the measurement error resulting from the signal crest factor, as
shown below:
Total Error ＝ Error (Sine wave) + Error (Crest factor) + Error (Bandwidth)
Error (Sine wave): error for sine wave.
Measurement Tutorial
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