Agilent Technologies 34901A User Manual page 344

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
R
L
V
test
R
L
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 =
Ci
2 π 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).
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) which is added to the measured voltage.
L
HI
DMM
LO
≈ C
i
250 pF
I
R
i
Ω
>10 G
V
ground
=
ground
+ Z
R
L
≈ Ω
10 M at 50 or 60 Hz
C
small compared to R
L
small compared to Z, and set the
L
)
ground
.
i
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