34405A User’s and Service Guide
53
Measurement Tutorial
3
True RMS AC Measurements
True RMS responding multimeters, like the Agilent 34405A, measure the
"heating" potential of an applied voltage. Power dissipated in a resistor is
proportional to the square of an applied voltage, independent of the
waveshape of the signal. This multimeter accurately measures true RMS
voltage or current, as long as the wave shape contains negligible energy
above the instrument’s effective bandwidth.
Note that the 34405A uses the same techniques to measure true RMS
voltage and true RMS current.
The multimeter's AC voltage and AC current functions measure the
AC
–
coupled
true RMS value. In this Agilent instrument, the “heating value”
of only the AC components
of the input waveform are measured (DC is
rejected). As seen in the figure above; for sinewaves, triangle waves, and
square waves, the
AC
–
coupled
and
AC+DC
values are equal, since these
waveforms do not contain a DC offset. However, for non–symmetrical
waveforms, such as pulse trains, there is a DC voltage content, which is
rejected by Agilent’s AC–coupled true RMS measurements. This can
provide a significant benefit.
Waveform Shape
Crest Factor
AC RMS
AC + DC RMS
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