Chapter 8 Tutorial

System Cabling and Connections

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.

RL

HI

Vtest

 

 

 

DMM

 

 

 

 

RL

LO

≈ Ci

250 pF

I

Ri >10 GΩ

Vground

Where:

RL = Lead resistance

 

 

 

 

Ri

= DMM isolation resistance

 

 

 

 

Ci

= DMM isolation capacitance

 

 

 

 

Vground = Ground noise voltage

 

Vground

 

 

I = Current flow caused by Vground

=

 

RL + Z

 

 

 

 

1

 

 

 

 

Z ≈ ZCi =

 

 

≈ 10 MΩ at 50 or 60 Hz

 

 

π f C

 

 

2

 

 

 

 

 

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

8

discussion of integration time).

 

 

 

 

 

 

 

 

 

 

 

 

 

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