Keysight 34450A Guia Del Usuario página 108

3 Measurement Tutorial
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.
Notice that crest factor is a composite parameter, dependent upon the
pulse-width and repetition frequency; crest factor alone is not enough to
characterize the frequency content of a signal.
108 Keysight 34450A User's Guide