Keithley 6430, Instruction Manual

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Measurement Considerations F-3
Low current measurements
Low current measurements are subject to a number of error sources that can have a serious
impact on measurement accuracy. First, the Remote PreAmp may cause measurement errors if
not connected properly. Making proper shielded connections is discussed in Section 2. The
voltage burden and input offset current of the ammeter may also affect the measurements. The
source impedance of the device under test will affect the noise performance of the Remote
PreAmp. Possible external error sources include leakage current from cables and
fi xtures, as
well as currents generated by effects such as triboelectric or piezoelectric.
Leakage currents and guarding
Leakage currents are generated by high resistance paths between the measurement circuit
and nearby voltage sources. These currents can considerably degrade the accuracy of low cur-
rent measurements. Some ways to reduce leakage currents are to use good quality insulators,
reduce humidity, and use guarding. Guarding can also be used to reduce the effect of shunt
capacitance in the measurement circuit.
One way to reduce leakage currents is to use good quality insulators when building the test
circuit. Some good quality insulators include Te fl on, polyethylene, and sapphire. Avoid materi-
als such as phenolics and nylon.
Humidity may also degrade low current measurements. The amount of water an insulator
absorbs will vary depending upon the insulator. It is best to choose an insulator on which water
vapor does not readily form a continuous
fi lm. Sometimes this is unavoidable if the material
being measured absorbs water easily, so it is best to make the measurements in an environmen-
tally controlled room. In some cases, an insulator may have ionic contaminants and, especially
in high humidity, a spurious current may be generated.
Another way to reduce leakage currents is to use guarding. A guard is a conductor connected
to a low impedance point in the circuit that is nearly at the same potential as the high imped-
ance lead being guarded. Guarding can isolate the high-impedance input lead of the Remote
PreAmp from leakage current due to voltage sources. The concepts of guarding are covered in
Section 5.