Keysight U2741A Guide D'utilisation page 60

3 Measurement Tutorial
AC current measurement errors (burden voltage)
Burden voltage errors, which apply to DC current, also apply to AC current
measurements. However, the burden voltage for AC current is larger due to the
digital multimeter's series inductance and your measurement connections. The
burden voltage increases as the input frequency increases. Some circuits may
oscillate when performing current measurements due to the digital multimeter's
series inductance and your measurement connections.
Low-level measurement errors
When measuring AC voltages less than 100 mV, be aware that these
measurements are especially susceptible to errors introduced by extraneous noise
sources. An exposed test lead acts as an antenna and a properly functioning
digital multimeter will measure the signals received. The entire measurement
path, including the power line, acts as a loop antenna. Circulating currents in the
loop create error voltages across any impedance in series with the digital
multimeter's input. For this reason, you should apply low-level AC voltages to the
digital multimeter through shielded cables. You should connect the shield to the
input LO terminal.
Make sure the digital multimeter and the AC source are connected to the same
electrical outlet whenever possible. You should also minimize the area of any
ground loops that cannot be avoided. A high-impedance source is more
susceptible to noise pickup than a low-impedance source. You can reduce the
high-frequency impedance of a source by placing a capacitor in parallel with the
digital multimeter's input terminals. You may have to experiment to determine the
correct capacitor value for your application.
Most extraneous noise is not correlated with the input signal. You can determine
the error as shown below
Voltage Measured
Correlated noise, while rare, is especially detrimental. Correlated noise always
adds directly to the input signal. Measuring a low-level signal with the same
frequency as the local power line is a common situation that is prone to this error.
60
(Equation 4):
2 2
= V + Noise
in
Keysight U2741A User's Guide
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