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Noise caused by ground loops
When measuring voltages in circuits where the internal DMM and the device-under-test are both referenced to a
common earth ground, a ground loop is formed. As shown below, any voltage difference between the two ground
reference points (V
ground
(V
) which is added to the measured voltage.
L
Where:
R
= Lead resistance
L
R
= DMM isolation resistance
i
C
= DMM isolation capacitance
i
V
= Ground noise voltage
ground
To minimize ground loop errors:
– If V
is a DC voltage, keep R
ground
– If V
is an AC voltage, keep R
ground
greater.
Keysight DAQ970A/DAQ973A User's Guide
) causes a current to flow through the LO measurement lead. This causes an error voltage
small compared to R
L
small compared to Z, and set the DMM's integration time to 1 PLC or
L
.
i
4 Measurement Tutorials
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