Low–Level Measurement Errors
AC voltage measurements less than 100 mV are especially susceptible to errors introduced by extraneous
noise sources. An exposed test lead acts as an antenna and a properly functioning DMM will measure the
signals received. The entire measurement path, including the power line, act as a loop antenna. Cir-
culating currents in the loop create error voltages across any impedances in series with the DMM's input.
Therefore, you should apply low–level AC voltages to the DMM through shielded cables, with the shield
connected to the input LO terminal.
Connect the DMM and the AC source to the same electrical outlet whenever possible. You should also min-
imize 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 DMM's input terminals. You may have to experiment to determ-
ine the correct capacitor 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 because it 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.
Common Mode Errors
Errors are generated when the multimeter's input LO terminal is driven with an AC voltage relative to
earth. The most common situation where unnecessary common mode voltages are created is when the
output of an AC calibrator is connected to the multimeter "backwards." Ideally, a multimeter reads the
same regardless of how the source is connected. Both source and multimeter effects can degrade this
ideal situation. Because of the capacitance between the input LO terminal and earth (approximately 200
pF), the source will experience different loading depending on how the input is applied. The magnitude of
the error is dependent upon the source's response to this loading.
The DMM's measurement circuitry, while extensively shielded, responds differently in the backward input
case due to slight differences in stray capacitance to earth. The DMM's errors are greatest for high–
voltage, high–frequency inputs. Typically, the DMM exhibits about 0.06% additional error for a 100 V, 100
kHz reverse input. You can use the grounding techniques described for DC common mode problems to
minimize AC common mode voltages.
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Series Operating and Service Guide
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Measurement Tutorial
