Agilent Technologies 34970A manual Sources of Error in AC Voltage Measurements

Chapter 8 Tutorial

Measurement Fundamentals

Sources of Error in AC Voltage Measurements

Many of the errors associated with dc voltage measurements also apply to ac voltage measurements. Additional errors unique to ac voltage measurements are described in this section.

Crest Factor Errors (non-sinusoidal inputs) A common misconception is that “since the internal DMM 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 the 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 page 360. 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 are shown in the specifications in chapter 9 on page 407. Note that the crest factor errors do not apply for input signals below 100 Hz when using the slow ac filter.

You can estimate the measurement error due to signal crest factor as shown below:

Total Error = Errorsine + Errorcrest factor + Errorbandwidth

Where:

Errorsine = DMM’s sinewave accuracy (see page 406)

Errorcrest factor = DMM’s crest factor (see page 407)

Errorbandwidth = Estimated bandwidth error as shown below:

–C.F. 2 x F Errorbandwidth = 4 π x BW

Where:

C.F. = Signal crest factor (see the table on page 360)

F = Fundamental input signal frequency

BW = DMM’s -3 dB bandwidth (1 MHz for the 34970A)

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