Operation
5-13
In this application, the DUTs are connected to the bank
inputs on the multiplexer and allow a large number of DUTs
to be switched through the matrix-digital I/O card. Also, the
instruments are connected to the columns on the matrix-dig-
ital I/O card. This particular configuration is best suited for
applications requiring a large number of DUTs to be con-
nected to several instruments. In other cases, the test config-
uration may call for a large number of instruments and few
DUTs. In those situations, the instruments would be con-
nected to the multiplexer inputs, and the DUTs would be
connected to the columns.
Measurement considerations
Many measurements made with the Model 7021 are subject
to various effects that can seriously affect low-level measure-
ment accuracy. The following paragraphs discuss these ef-
fects and ways to minimize them.
Path isolation
The path isolation is simply the equivalent impedance
between any two test paths in a measurement system. Ide-
ally, the path isolation should be infinite, but the actual
resistance and distributed capacitance of 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 paral-
lel with the equivalent resistance of the DUT, as shown in
Figure 5-11. 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 5-12. 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-digital I/O card is generally fixed by design,
there is one area where you do have control over the capaci-
tance in your system: the connecting cables. To minimize ca-
pacitance, keep all cables as short as possible.
Figure 5-11
Path isolation resistance
Figure 5-12
Voltage attenuation by path isolation resistance
R
E
DUT
DUT
R
PATH
V
R
IN
DUT
7021 Mux
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
R
E
DUT
DUT
R
PATH
E
OUT
R
PATH
R
DUT
R
PATH
+
=
E
DUT
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Summary of Contents for 7021
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