34410A/11A User’s Guide 101
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, 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.
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