True RMS AC Measurements
True RMS responding multimeters, like the Agilent 34401A, measure
the “heating” potential of an applied voltage. Unlike an “average
responding” measurement, a true
RMS measurement is used to
determine the power dissipated in a resistor. The power is proportional
to the square of the measured true
RMS voltage, independent of
waveshape. An average responding ac multimeter is calibrated to read
the same as a true
RMS meter for sinewave inputs only. For other
waveform shapes, an average responding meter will exhibit substantial
errors as shown below.
The multimeter’s ac voltage and ac current functions measure the
ac-coupled true
RMS value. This is in contrast to the ac+dc true RMS
value shown above. Only the “heating value” of the ac components of the
input waveform are measured (dc is rejected). For sinewaves, triangle
waves, and square waves, the ac and ac+dc values are equal since these
waveforms do not contain a dc offset. Non-symmetrical waveforms, such
as pulse trains, contain dc voltages which are rejected by ac-coupled
true
RMS measurements.
Chapter 7 Measurement Tutorial
True RMS AC Measurements
206
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