Measurement Considerations; Path Isolation - Keithley 2000-SCAN-20 Instruction Manual

3-12 Operation

Measurement considerations

Many measurements made with the Model 2000-SCAN-20 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

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.
Figure 3-4
Path isolation resistance
R
DUT
R
DUT
E
DUT
R
PATH
R
IN
Path isolation resistance forms a signal path that is in parallel with the equivalent resistance
of the DUT, as shown in
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
through these resistances by voltage sources in the system.
DUT
R
PATH
E
DUT
Scanner
Card
= Source Resistance of DUT
= Source EMF of DUT
= Path Isolation Resistance
= Input Resistance of Multimeter
Figure 3-4. For low-to-medium device resistance values, path isolation
V
R
IN
DMM
Figure 3-5. Also, leakage currents can be generated