34410A/11A/L4411A User’s Guide 109
Measurement Tutorial 4
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 using an
integration time of at least 10 power–line cycles (PLCs).
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.
As an example, consider a pulse train, one of the most challenging waveforms for a
multimeter. The pulse–width of that waveform largely determines its high–frequency
content. The frequency spectrum of an individual pulse is determined by its Fourier
Integral. The frequency spectrum of the pulse train is the Fourier Series that samples
along the Fourier Integral at multiples of the input pulse repetition frequency (prf).
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