User`s guide

263
Chapter 7 Tutorial
System Cabling and Connections
4
7
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
)
causes a current to flow through the LO measurement lead. This causes
an error voltage (V
L
) which is added to the measured voltage.
Where:
R
L
= Lead resistance
R
i
= DMM isolation resistance
C
i
= DMM isolation capacitance
V
ground
= Ground noise voltage
I = Current flow caused by V
ground
=
Z  Z
Ci
= 10 M at 50 or 60 Hz
V
L
=
To minimize ground loop errors:
•If V
ground
is a DC voltage, keep R
L
small compared to R
i
.
•If V
ground
is an AC voltage, keep R
L
small compared to Z, and set the
DMM’s integration time to 1 PLC or greater (see page 120 for a
discussion of integration time).
DMM
HI
LO
R
L
R
L
V
ground
I
V
test
C
i
250 pF
R
i
>10 G
V
ground
R
L
Z+
---------------------
1
2fC
-------------
IR
L

34970A Refresh UG.book Page 263 Wednesday, February 17, 2010 12:34 PM