34410A/11A/L4411A User’s Guide 105
Measurement Tutorial 4
Noise Caused by Magnetic Loops
If you are making measurements near magnetic fields, take caution to avoid inducing
voltages in the measurement connections. You should be especially careful when
working near conductors carrying large currents. Use twisted–pair connections to the
multimeter to reduce the noise pickup loop area, or dress the test leads as close together
as possible. Loose or vibrating test leads will also induce error voltages. Tie down test
leads securely when operating near magnetic fields. Whenever possible, use 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.
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