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Chapter 7 Tutorial
Measurement Fundamentals
True RMS AC Measurements True RMS responding multimeters
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 internal DMM’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 component of the
input waveform is 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.
An ac-coupled true RMS measurement is desirable in situations where
you are measuring small AC signals in the presence of large DC offsets.
For example, this situation is common when measuring AC ripple
present on DC power supplies. There are situations, however, where you
might want to know the ac+dc true RMS value. You can determine this
value by combining results from DC and AC measurements as shown
below. You should perform the DC measurement using at least 10 power
line cycles of integration (6½ digit mode) for best AC rejection.
AC DC
+
AC
2
DC
2
+ =
34970A Refresh UG.book Page 282 Wednesday, February 17, 2010 12:34 PM