34410A/11A/L4411A User’s Guide 103
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.
UG_ED5.book Page 103 Thursday, March 1, 2012 11:28 AM
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