Tutorial 7
Keysight 34970A/34972A User’s Guide 295
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.
To Next Page
Loading...
