Keithley 7013-C, Instruction Manual

Page: 41 / 71

Operation
4-10
4.5 Measurement considerations
Many measurements made with the Model 7013 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 infinite, but the
actual resistance and distributed capacitance of cables
and connectors results in less than infinite 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 4-9. 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 4-10. Also,
leakage currents can be generated through these resis-
tances by voltage sources in the system.
Any differential isolation capacitance affects DC mea-
surement settling time as well as AC measurement
accuracy. Thus, it is often important that such capaci-
tance be kept as low as possible. Although the distrib-
uted capacitance of the switch card is generally fixed
by design, there is one area where you do have control
over the capacitance in your system; the connecting
cables. To minimize capacitance, keep all cables as
short as possible.
R
DUT
E
DUT
R
PATH
V R
IN
DUT
7013
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 4-9
Path isolation resistance
R
DUT
E
DUT
R
PATH
E
OUT
R
PATH
R
DUT
R
PATH
+
=
E
DUT
Figure 4-10
Voltage attenuation by path isolation resistance