General Measurement Considerations; Ground Loops - Keithley 2500 User Manual

Models 2500 and 2502 User's Manual

General measurement considerations

The following measurement considerations apply to all precision measurements.

Ground loops

Ground loops that occur in multiple-instrument test setups can create error signals that
cause erratic or erroneous measurements. The configuration shown in
duces errors in two ways. Large ground currents flowing in one of the wires will encounter
small resistances, either in the wires, or at the connecting points. This small resistance
results in voltage drops that can affect the measurement. Even if the ground loop currents
are small, magnetic flux cutting across the large loops formed by the ground leads can
induce sufficient voltages to disturb sensitive measurements.
To prevent ground loops, instruments should be connected to ground at only a single point,
as shown in
line ground. Experimentation is the best way to determine an acceptable arrangement. For
this purpose, measuring instruments should be placed on their lowest ranges. The configu-
ration that results in the lowest noise signal is the one that should be used.
Figure F-2
Power line ground loops
Instrument 1
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Figure F-3. Note that only a single instrument is connected directly to power
Signal Leads
Instrument 2
Ground
Loop
Current
Power Line Ground
Measurement Considerations
Instrument 3
F-7
Figure F-2 intro-