56 34405A User’s and Service Guide
3 Measurement Tutorial
Noise Caused by Ground Loops
When measuring voltages in circuits where the multimeter 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
measurement leads. This causes noise and offset voltage (usually
power–line related), which are added to the measured voltage.
The best way to eliminate ground loops is to isolate the multimeter from
earth by not grounding the input terminals. If the multimeter must be
earth–referenced, connect it and the device under test to the same
common ground point. Also connect the multimeter and device under test
to the same electrical outlet whenever possible.
Ideal
Meter
R
L
R
L
HI
LO
V
ground
V
test
R
i
> 10 GΩ
R
L
= Lead Resistance
R
i
= Multimeter Isolation Resistance
V
ground
= Voltage Drop on Ground Bus
34405A users guide.book Page 56 Saturday, September 2, 2006 3:38 PM
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