Operation
4-14
The effects of channel resistance can be minimized by fol-
lowing a few basic rules:
• When measuring resistance, use only the four-wire
measurement technique for resistances less than
100k
Ω
.
• When measuring voltage, use an instrument with high
input impedance.
• Whenever possible, keep the current flowing through
the card to an absolute minimum.
4.5.3 Path isolation
The path isolation is simply the equivalent impedance be-
tween any two test paths in a measurement system. Ideally,
the path isolation should be infinite, but the actual resistance
and distributed capacitance of the card, cables, and connec-
tors 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 Fig-
ure 4-11. 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 4-12. Also,
leakage currents can be generated through these resistances
by voltage sources in the system.
Any differential isolation capacitance affects DC measure-
ment settling time as well as AC measurement accuracy.
Thus, it is often important that such capacitance be kept as
low as possible. Although the distributed capacitance of the
multiplexer 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 connecting cables as short as possible.
Figure 4-11
Path isolation resistance
R
E
DUT
DUT
R
PATH
V
R
IN
DUT
MUX
Card
Measuring
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 4-12
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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