True Rms Ac Measurements - HP 34401A User Manual

Chapter 7 Measurement Tutorial

True RMS AC Measurements

True RMS AC Measurements
True responding multimeters, like the HP 34401A, measure the
RMS
"heating" potential of an applied voltage. Unlike an "average responding"
measurement, a true measurement is used to determine the power
RMS
dissipated in a resistor. The power is proportional to the square of the
measured true voltage, independent of waveshape. An average
RMS
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 value. This is in contrast to the ac+dc true
RMS 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 measurements.
RMS
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