Noise Caused by Ground Loops
When measuring voltages in circuits where the meter 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) causes a current to flow
through the LO measurement lead. This causes an error voltage, V
L
,
which is added to the measured voltage.
To minimize ground loop errors:
• If V
ground
is a dc voltage, keep R
L
small compared to Ri
.
• If V
ground
is an ac voltage, keep R
L
small compared to Z
,
and set the
meter’s integration time (NPLC) to 1 or greater. See page 58.
R
L
=
Lead
Resistance
R
i
=
Multimeter
Isolation
Resistance
C
i
=
Multimeter
Isolation
Capacitance
V
ground
=
Ground
Noise
Voltage
I
=
V
ground
caused
current
flow
=
V
ground
R
L
+
Z
Z
=
R
i
∗
Z
C
i
R
i
+
Z
C
i
V
L
=
I
∗
R
L
Ideal
Meter
7
Chapter 7 Measurement Tutorial
Measurement Techniques and Sources of Error
253
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