Model 4200A-SCS Source-Measure Unit (SMU) User's Manual
Section 6: Optimizing SMU measurements
4200A-SMU-900-01 Rev. A December 2020
6-5
Leakage currents
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-current
measurements.
Cable leakage currents are a common source of leakage. Typically, insulation resistance between
conductors in the type of triaxial cables supplied with the SMUs and preamplifiers is approximately
1
PΩ
(10
15
Ω
)
If the cables were used in an unguarded configuration, leakage current would flow
through the cable insulation, affecting the measurement. Properly connecting the triaxial cables to the
SMU or preamplifier automatically drives the inner cable shield at guard potential, minimizing the
effects of cable leakage currents. See
(on page 5-1) for details.
Methods to reduce leakage currents include:
•
Use good quality insulators, such as Teflon or polyethylene, in the test fixture.
•
Reduce the humidity of the test environment. Insulators and the test circuit may absorb water,
causing spurious currents to be generated.
•
Use guarding in the test fixture to isolate the high-impedance nodes from leakage current due to
(on page 5-4) for details.
Generated currents
Any extraneous generated currents in the test system add to the expected current, which can cause
errors. Currents can be internally generated, as in the case of preamplifier input offset current, or they
can come from external sources such as insulators and cables. The following paragraphs discuss the
various types of generated currents. The following table summarizes the typical ranges of a number
of generated currents.
Typical generated currents
Effect
Generated current range
Triboelectric
1 fA to 10 nA
Mechanical stress (Teflon)
1 fA to 1 pA
Mechanical stress (ceramics)
100 aA to 100 fA
Clean epoxy circuit board
100 fA
Dirty epoxy circuit board
100 pA
