HP 34401A, User Manual

Page: 205 / 236

True RMS AC Measurements
True
RMS
responding multimeters, like the HP 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