Keithley 6485, User Manual

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B-2 General Measurement Considerations Model 6485 and 6487 User’s Manual
Measurement considerations
The following measurement considerations apply to all precision measurements.
Table 2-1 lists all measurement considerations and indicates where to find detailed infor-
mation on them.
For additional measurement considerations, see Appendix C of the Model 6485 Instruc-
tion Manual or Appendix G of the Model 6487 Reference Manual. For comprehensive
information on all measurement considerations, refer to the Low Level Measurements
handbook, which is available from Keithley.
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
Figure B-1 intro-
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.
Figure B-1
Power line ground loops
Instrument 1 Instrument 2 Instrument 3
Signal Leads
Ground
Loop
Current
Power Line Ground
To prevent ground loops, instruments should be connected to ground at only a single point
as shown in
Figure B-2 . Note that only a single instrument is connected directly to power
line ground. Experimentation is the best way to determine an acceptable arrangement. For