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98640A Analog Input Interface
The voltage input on the A-to-D card proper is found at location D-18 on the schematic diagram.
This is where the voltage from the wire termination assembly enters the A-to-D card. A typical in-
put is shown in detail in figure 3-2. The input voltages enter through 1000-ohm resistors contained
in resistor arrays U61 and U71. These packs each cc;ntain 8 series resistors, and limit current to the
overvoltage protection that follows. The over voltage protection consists of two rail voltages
(+
10 volts
and -10 volts), defined by diode CR 1 for the positive side and by diode CR2 for the negative side.
Each input signal path has a reverse-biased diode connecting it to each of the rail voltages. These
diodes are contained in diode arrays U62 and U72.
+12
INPUT
lKO
TO
ANALOG
MUX
Figure 3-2. Input circuit
-12
Resistors R5 and R6 maintain current through the zener diodes to keep them in the breakdown state.
In the case of an overvoltage condition, the input voltage will exceed the zener value plus 0.7 volts.
This will forward bias the appropriate diode in one of the arrays and allow current to flow through
the zener diode. At this time the voltage at the diode array will not exceed approximately 10.7 volts
and the rest of the voltage will be dropped across the lOaD-ohm resistor. This circuit protects the in-
puts to the analog multiplexer (mux), U64, from overvoltage conditions at the inputs.
NOTE
The limitation on the amount of over voltage depends on
power dissipated by the 1000-ohm, 1/ 4-watt resistor: if
too much voltage is applied, the resistor will fail.
All of the analog inputs are protected in this way.
Channel Selection
Channel selection is accomplished by U64, an analog multiplexer (mux). This mux switches one of
eight differential channels into the programmable gain amplifier (POA). Its operation is controlled by
address lines AA 1, AA2, and AA3, on pins 17, 16, and 15 of U64. The differential outputs, across pins
2 and 28 go directly into the positive inputs of op amps U44 and U54.
3-6
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