Keysight DAQ970A Guia Del Usuario página 235

4 Measurement Tutorials
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 for best AC rejection.
Keysight DAQ970A/DAQ973A User's Guide 235