Keithley 2750 User Manual page 414

Model 2750 Multimeter/Switch System User's Manual
Ground loops
When two or more instruments are connected together, care must be taken to avoid
unwanted signals caused by ground loops. Ground loops usually occur when sensitive
instrumentation is connected to other instrumentation with more than one signal return
path such as power line ground. As shown in
current to flow through the instrument LO signal leads and then back through power line
ground. This circulating current develops a small but undesirable voltage between the LO
terminals of the two instruments. This voltage will be added to the source voltage, affect-
ing the accuracy of the measurement.
Figure E-2
Power line ground loops
Instrument 1
Signal Leads
Instrument 2
Ground
Loop
Current
Power Line Ground
Measurement Considerations
Figure E-2, the resulting ground loop causes
Instrument 3
E-7