Low Current Measurements; Leakage Currents; Sources Of Leakage Currents; Cable Leakage Currents - Keithley 4200-SCS Reference Manual

Model 4200-SCS Reference Manual
Figure 5-15
Undesirable and desirable current measurement configurations for a BJT
Undesirable: Low-frequency
oscillation may occur.
NOTE

Low current measurements

Low-current measurements made with a SMU or preamp are subject to a number of error sources
that can have a serious impact on measurement accuracy. The following paragraphs discuss
these low current measurement considerations.

Leakage currents

Sources of 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

Typically, insulation resistance between conductors in the type of triax cables supplied with the
SMUs and preamps is approximately 1 PΩ (10
configuration, leakage current would flow through the cable insulation, affecting the measurement.
Properly connecting the triax cables to the SMU or preamp automatically drives the inner cable
shield at guard potential, minimizing the effects of cable leakage currents. See
beginning of this section for details.
4200-901-01 Rev. S / May 2017
Low LtdAuto
range at Collector
results in low
minimum current
range and high
maximum series
resistance.
Common forcing
function at Emitter
behaves like
ground. Therefore,
no current
measurement can
be set. Minimum
resistance is in
series with the
Emitter.
For instructions on configuring KITE ITMs, refer to
in Section 6.
Return to Section Topics
Section 5: Source-Measure Concepts
Desirable: Helps avoid low-
frequency oscillation.
Configuring the Project Plan ITMs
15
Ω). If the cables were used in an unguarded
Low LtdAuto range at
Collector results in low
minimum current range
and high maximum series
resistance.
Voltage Bias
forcing function
allows current
measurement
range to be set.
Identical LtdAuto
range at Emitter
ensures the same
(or, worst case,
comparable) series
resistance.
Guarding near the
5-19