Noise Caused by Ground Loops When measuring voltages in circuits
where the internal DMM 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 (Vground)
causes a current to flow through the LO measurement lead. This causes
an error voltage (VL) which is added to the measured voltage.
Where:
RL = Lead resistance
Ri = DMM isolation resistance
Ci = DMM isolation capacitance
Vground = Ground noise voltage
I = Current flow caused by Vground = Vground
RL + Z
Z ≈ ZCi = 1
2 π f C ≈ 10 MΩ at 50 or 60 Hz
VL = I x RL
To minimize ground loop errors:
•If Vground is a dc voltage, keep RL small compared to Ri.
•If Vground is an ac voltage, keep RL small compared to Z, and set the
DMM’s integration time to 1 PLC or greater (see page 103 for a
discussion of integration time).
Vtest
RL
DMM
≈ Ci
250 pF
RL
I
Vground
Ri
>10 GΩ
LO
HI
8
Chapter 8 Tutorial
System Cabling and Connections
341