the ADC0801. It is important that the voltage levels that drive
the auto-zero resistors be constant. Also, for symmetry, a
logic swing of 0V to 5V is convenient. To achieve this, a
CMOS buffer is used for the logic output signals of Port B
and this CMOS package is powered with a stable 5V source.
Buffer amplifier A1 is necessary so that it can source or sink
the D/A output current.
Functional Description (Continued)
any output of Port B will source current into node VX thus
raising the voltage at VX and making the output differential
more negative. Conversely, a logic “0” (0V) will pull current
out of node VX and decrease the voltage, causing the differ-
ential output to become more positive. For the resistor val-
±
ues shown, VX can move 12 mV with a resolution of 50 µV,
1
which will null the offset error term to
⁄4 LSB of full-scale for
DS005671-91
=
Note 26: R2 49.5 R1
Note 27: Switches are LMC13334 CMOS analog switches.
Note 28: The 9 resistors used in the auto-zero section can be 5% tolerance.
±
FIGURE 18. Gain of 100 Differential Transducer Preamp
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