Keysight 34450A Guia Del Usuario página 117

Measurement Tutorial 3
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
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 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
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 impedances in series with the multimeter's input.
For this reason, you should apply low-level AC voltages to the multimeter through
shielded cables. You should connect the shield to the input LO terminal.
Make sure the 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 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.
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.
Keysight 34450A User's Guide 117