50
34405A User’s and Service Guide
3
Measurement Tutorial
vibrating test leads will also induce error voltages. Tie down test leads
securely when operating near magnetic fields. Whenever possible, utilize
magnetic shielding materials or increased distance from magnetic sources.
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 G
Ω
R
L
= Lead Resistance
R
i
= Multimeter Isolation Resistance
V
ground
= Voltage Drop on Ground Bus
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