Keithley 7012-S, Instruction Manual, Page: 59 / 93

Operation
4-13
4.5 Measurement considerations
Many measurements made with the Model 7012 are
subject to various effects that can seriously affect low-
level measurement accuracy. The following para-
graphs discuss these effects and ways to minimize
them.
4.5.1 Path isolation
The path isolation is simply the equivalent impedance
between any two test paths in a measurement system.
Ideally, the path isolation should be in
Þ nite, but the ac-
tual resistance and distributed capacitance of cables
and connectors results in less than in
Þ nite path isola-
tion values for these devices.
Path isolation resistance forms a signal path that is in
parallel with the equivalent resistance of the DUT, as
shown in Figure 4-10. For low-to-medium device resis-
tance values, path isolation resistance is seldom a con-
sideration; however, it can seriously degrade
measurement accuracy when testing high-impedance
devices. The voltage measured across such a device, for
example, can be substantially attenuated by the volt-
age divider action of the device source resistance and
path isolation resistance, as shown in Figure 4-11. Also,
leakage currents can be generated through these resis-
tances by voltage sources in the system.
Figure 4-9
Common-emitter characteristics of an NPN silicon
Transistor
0 1 2 3 4 5
10
8
4
2
6
V , volts
CE
I
, ma
c
+80 µa
+60 µa
+40µa
+20 µa
I
= 0
B
Figure 4-10
Path isolation resistance
R
E
DUT
DUT
R
PATH
V
R
IN
DUT
Matrix
Card
Measure
Instrument
= Source Resistance of DUT
= Source EMF of DUT
= Path Isolation Resistance
= Input Resistance of Measuring Instrument
R
DUT
E
DUT
R
PATH
R
IN
Figure 4-11
Voltage attenuation by path isolation resistance
R
E
DUT
DUT
R
PATH
E
OUT
R
PATH
R
DUT
R
PATH
+
=
E
DUT
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