Fluke 8842a User Manual download pdf (Page 104)

Measurement Tutorial

AC VOLTAGE AND CURRENT MEASUREMENT

4-13

measurements, for instance, cannot be made with dc coupling. Remember, however, that

when the 8842A measures signals with the ac functions, the reading on the display does

not include the dc component (if one exists). For example, consider Figure 4-8, which

shows a simple ac signal riding on a dc level. The VAC function would measure the ac

component only.

f4-08.wmf

Figure 4-8. Combined AC and DC Measurement

4-20. Combined AC and DC Measurements

The 8842A can be used to evaluate the true rms value of waveforms such as the one

shown in Figure 4-8, which includes both ac and dc components. First, measure the rms

value of the ac component using the VAC function. Next, measure the dc component

using the VDC function. Finally, calculate the total rms value as follows:

RMSV

DCAC

VV +=

4-21. Bandwidth

Bandwidth defines the range of frequencies to which an instrument can respond

accurately. The accuracy of the 8842A is specified for sinusoidal waveforms up to 100

kHz, or for nonsinusoidal waveforms with frequency components up to 100 kHz. The

small-signal bandwidth (the frequency at which the response is 3 dB down) is typically

around 300 kHz.

For signals with components greater than 100 kHz, the measurement accuracy is reduced

because of frequency bandwidth and slew-rate limitations. Because of this, accuracy may

be reduced when measuring signals with fast rise times, such as high-frequency square

waves or switching supply waveforms. As a rule of thumb, an ac voltage input signal is

within the bandwidth limitations if the rise time is longer than 2 us, and within the slew-

rate limitations if the input slew rate is slower than (1V/µs)x(full scale of range).

4-22. Zero-Input VAC Error

If the 8842A input terminals are shorted while the VAC function is selected, the 8842A

displays a non-zero reading (typically less than 80 digits in the highest four ranges, and

less than 300 digits in the 200 mV range). Such readings are due to random noise

combined with the inherent nonlinear response of computing-type rms converters to very

small input signals.

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Fluke 8842a User Manual Page 104