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9 8640A Analog Input Interface
10K
V OU ' -
10K
2
7
VOUT
TO SAMPLE
AND HOLD
V OUT
+
10K
Figure 3-5. Differential-to-single-ended converter
Absolute Value Circuit
The absolute value circuit takes the voltage coming out of the S/H circuit and makes sure that it is
positive when it goes to the analog-to-digital converter (ADC). This circuit also generates a polarity
bit to
indicat~~
when a negative voltage has been converted to positive. This polarity bit shows up as
the sign bit (bit 12) in the output data register. This circuit is made up of two op amps (U22 and
U23), a handful of diodes (CR3, CR4, and CR6), a capacitor (C43), and 4 resistors (R 7, R15, and two
resistors in U24). The circuit is shown schematically in figure 3-6.
The output of the absolute value circuit is driven by the more positive of the two op amps. One of the
op amps (U22) is a unity gain buffer, and the other (U23) is a unity gain inverter. Thus, one of the
outputs will always be positive. Diodes CR3 and CR4 separate the negative and positive outputs, and
cause the positive output to
be
sent to the ADC.
Resistors R 7 and R 15, and diode CR6 are used to generate a high logic signal if the analog input signal
is negative. The pull-up resistor (R
7)
will pull the polarity signal high (+5 volts) whenever the output
of the inverting op amp (U23) is greater than 0; this will be the case whenever the analog input to
3-11
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