Noise Caused by Ground Loops When measuring voltages in circuits where the multimeter and the
device-under-test are both ref erenced to a common earth grou nd,
a “ground loop” is formed. As shown below, any voltage difference
between the two ground reference points (Vground) causes a current to
flow through the measurement leads. This causes erro rs, such as noise
and offset voltage (usually power-line related), which are added to the
measured voltage.
The best way to eliminate ground loops is to maintain the multimeter’s
isolation from earth; do not connect the input term i na l s t o ground.
If the multimeter must be earth-referenced, be sure to conn ect it ,
and the device-under-test, to the same common ground point. This will
reduce or eliminate any voltage difference between the devices.
Also make sure the multimeter and device-under-test are connected to
the same electrical outlet whenever possible.
Ideal
Meter
RL
Ri >10 GΩ
Vground
RL
Vtest
HI
LO
RL = lead resistance
Ri = multimeter isolation resistance
Vground = voltage drop on ground bus
Chapter 7 Measurement Tutorial
Noise Caused by Ground Loops
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