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Chapter 8 Tutorial
System Cabling and Connections
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
causes a current to flow through the LO measurement lead. This causes
an error voltage (V
) which is added to the measured voltage.
L
R
L
HI
V
test
R
L
LO
I
V
ground
Where:
R
= Lead resistance
L
R
= DMM isolation resistance
i
C
= DMM isolation capacitance
i
V
= Ground noise voltage
ground
I = Current flow caused by V
1
Z
Z
=
10 M at 50 or 60 Hz
Ci
C
2
f
V
= I x R
L
L
To minimize ground loop errors:
If V
is a dc voltage, keep R
ground
If V
is an ac voltage, keep R
ground
DMM
's integration time to 1 PLC or greater (see page 103 for a
discussion of integration time).
DMM
C
i
250 pF
R
i
>10 G
V
ground
=
ground
R
Z
L
small compared to R
L
small compared to Z, and set the
L
)
ground
.
i
8
341
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