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3
Measurement Tutorial
Table 3-3 Waveform shapes and their parameters
Waveform shape
56
Crest factor (CF)
T
------- -
t
p
1
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
Agilent'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
TrueRMS
AC RMS
2
V
1
⎛
⎞
------- -
------- -
×
1
–
⎝
⎠
CF
CF
V
(Equation
2
2
=
AC
+
DC
AC+DC RMS
V
------- -
CF
V
):
(1)
U2741A User's Guide
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