Keithley 2000-Scan, Instruction Manual, Page: 54 / 86

Measurement considerations
Many measurements made with the Model 2000-SCAN are subject to various effects that can
seriously affect low-level measurement accuracy. The following paragraphs discuss these effects
and ways to minimize them.
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 infinite, but the actual resistance and
distributed capacitance of cables and connectors results in less than infinite path isolation 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 3-5 . For low-to-medium device resistance values, path isolation
resistance is seldom a consideration; 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 voltage divider action of the device source resistance and
path isolation resistance, as shown in
Figure 3-6 . Also, leakage currents can be generated
through these resistances by voltage sources in the system.
R
E
DUT
DUT
R
PATH
V R
IN
DUT
Scanner
Card
DMM
= Source Resistance of DUT
= Source EMF of DUT
= Path Isolation Resistance
= Input Resistance of Multimeter
R
DUT
E
DUT
R
PATH
R
IN
Figure 3-5
Path isolation
resistance
3-20 Operation