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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 will act as an antenna
and a properly functioning multimeter will measure the signals
received. The entire measurement path, including the power line, act as
a loop antenna. Circulating currents in the loop will 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
will always add 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.
Voltage Measured = V
in
2
+ Noise
2
√
7
Chapter 7 Measurement Tutorial
Low-Level Measurement Errors
211