Noise Caused By Ground Loops - HP 34401A User Manual

Chapter 7 Measurement Tutorial

Noise Caused by Ground Loops

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
flow through the measurement leads. This causes errors, 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 terminals to ground.
If the multimeter must be earth-referenced, be sure to connect 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.
R
L
V
test
R
L
V
ground
R = lead resistance
L
R = multimeter isolation resistance
i
Vground = voltage drop on ground bus
202
) causes a current to
ground
HI
Ideal
Meter
LO
R >10 G
i