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98640A Analog Input Interface
Figure 2-·4 shows the two recommended hook ups for a reference channel. Any channel may serve as
a reference channel. If you determine by experiment that the offset voltages on an input channel are
acceptable in your application, you can use all 8 channels for making measurements.
Figure 2-·5 shows a few ways to wire a channel so a voltage source can be measured. Appendix A
shows a possible circuit application. There are many circuits into which the interface can be added to
make voltage measurements. We will not try to list them here.
Figure 2-6 has examples of hook -ups which are not recommended. The two problems the wiring ar-
rangements in figure 2-6 cause are ground-loops or over voltage on an input.
A ground-loop will
contribute unwanted noise to the input voltage. Also, if the difference in potential between chassis
grour:d and the voltage source's ground is high enough, the current limiting resistor on the wire ter-
mination assembly may be damaged.
Once you have finished adding the wires to the wire t.ermination assembly and the voltage sources you
may '''Vish to use cable ties to form the wires into bundles.
If you have several wire termination as-
semblies you can fabricate several wire bundles allo\\ ing a rapid change of the
7
or 8 circuits you are
making your measurements from.
+
+
CHANNEL
11
CHANNEL
11
9
GND TERMINAL
r-h
GND TERMINAL
I
rT7
1"
I
I
I
Figure 2-4. Wiring a reference channel.
CAUTION
Read Appendix A before you configure your system.
The resistance between any input and ground is low
when the A -to-D card's host is powered down.
2-10
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