C-2 General Measurement Considerations Model 6485 Picoammeter Instruction Manual
Measurement considerations
The following measurement considerations apply to all precision measurements. Table 2-5
lists all measurement considerations, and indicates where to find detailed information on
them.
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 C-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 C-1
Power line ground loops
To prevent ground loops, instruments should be connected to ground at only a single point
as shown in Figure C-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
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. A convenient
way to make this connection uses the ground link at the rear of the 6485.
Instrument 1 Instrument 2 Instrument 3
Signal Leads
Ground
Loop
Current
Power Line Ground
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