Keithley 6482, Reference Manual

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D-6 Return to
Section Topics
6482-901-01 Rev. A / August 2012
Appendix D: Measurement Considerations Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual
Figure D-1
Voltage burden
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
Figure D-2 introduces 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
Figure D-3 . Note that only a single instrument is connected directly to power 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 configuration that results in
the lowest noise signal is the one that should be used.
Figure D-2
Power line ground loops
R
L
I
L
= < 1mV
Model 6482
I
M
V
S
V
B
I
M
=
V
S
– V
B
R
L
Instrument 1 Instrument 2 Instrument 3
Signal Leads
Ground
Loop
Current
Power Line Ground