Keithley 7999-2, Instruction Manual

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Operating Considerations 3-3
problems, never touch the connector insulating material. In addition, the multiplexer card
should be used only in clean, dry environments to avoid contamination.
If the connector insulators should become contaminated, either by inadvertent touching, or
from air-borne deposits, they can be cleaned with a cotton swab dipped in clean methanol.
After thoroughly cleaning, they should be allowed to dry for several hours in a low-humidity
environment before use, or they can be dried more quickly using dry nitrogen.
Path isolation
The path isolation is simply the equivalent impedance between any two test paths in a mea-
surement system. Ideally, the path isolation should be infinite, but the actual resistance and dis-
tributed 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 3-1. 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 3-2. Also, leakage currents can be generated
through these resistances by voltage sources in the system.
DUT MUX
Card
Measure
Instrument
V
R
DUT
E
DUT
R
PATH
R
IN
= 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 3-1
Path isolation
resistance