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300 Keysight 34970A/34972A User’s Guide
Low-Level AC 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 the internal DMM 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 instrument’s input. For this
reason, you should apply low-level AC voltages to the instrument through
shielded cables. You should also connect the shield to the input LO terminal.
Be sure to 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 instrument’s input terminals. You may
have to experiment to determine the correct capacitance 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 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.
You should use caution when switching high-level and low-level signals on the
same module. It is possible that high-level charged voltages may be discharged
onto a low-level channel. It is recommended that you either use two different
modules or separate the high-level signals from the low-level signals with an
unused channel connected to ground.
Measurements Below Full Scale
You can make the most accurate AC measurements when the internal DMM is at
full scale of the selected range. Autoranging occurs at 10% and 120% of full scale.
This enables you to measure some inputs at full scale on one range and 10% of
full scale on the next higher range. Note that the measurement accuracy will be
significantly different for the two cases. For highest accuracy, you should use
manual ranging to select the lowest range possible for the measurement.
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