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98640A Analog Input Interface
24
25tS~
281<
2
23
1K
8k
3
·22
255~
28k
4
21
31~
10K
5
20
1K
U24
10K
6
19
31.&1<
7
18
10K
10K
~
8
17
10K
10K
9
16
10
15
50/\
50/\
11
-
14
12
I
13
Figure 3-4. Precision resistor network (U24)
OTHER GAINS
The situation is slightly more complicated for gains greater than 1, since the resistor strings actually
connect the two op amps together. As long as there is a difference between the two inputs, the op
amps will have to adjust their outputs in order to equalize the voltages at their respective + and - in-
puts. When the circuit has reached equilibrium, the difference between the op amp output voltages
will have been amplified by the appropriate gain factor. Let's look at an example.
Example
Assume that your A-to-D card is connected to a device that is supplying +3.01
volts to the + input of a channel and +3.00 volts to the - input. Assume also
tha.t you are measuring that voltage at a gain of 512. The + input (pin 3) of op
amp U 54 will see a voltage of + 3.0 1 volts, and the op amp will adjust its output
until the - input (pin 2) sees the same voltage. Similarly, the + input of U 44
will see a voltage of + 3.00 volts, and the output will adjust until the - input
sees the same voltage.
3-9
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