Noise Caused by Ground Loops
When measuring voltages in circuits where the multimeter and the DUT are referenced to a common earth ground, a
"ground loop" is formed. As shown below, any voltage difference between the two ground reference points (Vground)
causes 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 DUT to one common ground point. Also connect the
multimeter and DUT to the same electrical outlet whenever possible.
Agilent Truevolt Series DMM Operating and Service Guide 87
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