Keysight U2741A Guide D'utilisation page 56

3 Measurement Tutorial
The digital multimeter's AC voltage and AC current functions measure the
AC-coupled true RMS value, where only the "heating value" of only the AC
components of the input waveform is measured while the DC component is
rejected. As seen in the figure above; for sine waves, triangular waves, and square
waves, the AC-coupled and AC+DC values are equal, since these waveforms do
not contain a DC offset. However, for non-symmetrical waveforms, such as pulse
trains, there is a DC voltage content, which is rejected by Keysight's AC-coupled
true RMS measurements. This can provide a significant benefit. An AC-coupled
true RMS measurement is desirable when you are measuring small AC signals in
the presence of large DC offsets.
A good example is measuring AC ripples that are present in 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
AC DC
(
For the best AC noise rejection, you should perform the DC measurement at
20 NPLC.
True RMS accuracy and high-frequency signal content
A common misconception is that "since an AC digital multimeter is true RMS, its
sine wave accuracy specifications apply to all waveforms." Actually, the shape of
the input signal can dramatically affect the measurement accuracy especially
when that input signal contains high-frequency components which exceed the
instrument's bandwidth.
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+ = AC
)
TrueRMS
(Equation 1):
2 + DC 2
Keysight U2741A User's Guide
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