User`s guide

88 34450A User’s Guide
3 Measurement Tutorial
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. 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:
For the best AC noise rejection, you should perform the DC
measurement at s- mode.
True RMS Accuracy and High- Frequency Signal Content
A common misconception is that “since an AC multimeter is
true RMS, its sine wave accuracy specifications apply to all
waveforms.” Actually, the shape of the input signal can
dramatically affect measurement accuracy, for any
multimeter, especially when that input signal contains
high- frequency components which exceed the instrument’s
bandwidth. Error in RMS measurements arise when there is
significant input signal energy at frequencies above the
multimeter’s bandwidth.
Estimating High- Frequency (Out- of- Band) Error
A common way to describe signal waveshapes is to refer to
their “Crest Factor”. Crest factor is the ratio of the peak
value to RMS value of a waveform. For a pulse train, for
example, the crest factor is approximately equal to the
square root of the inverse of the duty cycle.