Agilent Technologies 34401A Crest Factor Errors non-sinusoidal inputs, Ac + dc = √ ac2 + dc2

Chapter 7 Measurement Tutorial

Crest Factor Errors (non-sinusoidal inputs)

An ac-coupled true RMS measurement is desirable in situations where 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. You should perform the dc measurement using at least 10 power line cycles of integration (6 digit mode) for best ac rejection.

ac + dc = √ ac2 + dc2

Crest Factor Errors (non-sinusoidal inputs)

A common misconception is that “since an ac multimeter is true RMS, its sinewave accuracy specifications apply to all waveforms.” Actually, the shape of the input signal can dramatically affect measurement accuracy. A common way to describe signal waveshapes is 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 as shown in the table on the previous page. In general, the greater the crest factor, the greater the energy contained in higher frequency harmonics. All multimeters exhibit measurement errors that are crest factor dependent. Crest factor errors for the Agilent 34401A are shown in the specifications in chapter

8.Note that the crest factor errors do not apply for input signals below

100Hz when using the slow ac filter.

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