Specifications

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