ADC0803 datasheet(34/41 Pages) NSC

34 / 41 page

Functional Description (Continued)

any output of Port B will source current into node V

X thus

raising the voltage at V

X and making the output differential

more negative. Conversely, a logic “0” (0V) will pull current

out of node V

X and decrease the voltage, causing the differ-

ential output to become more positive. For the resistor val-

ues shown, V

X can move ±12 mV with a resolution of 50 µV,

which will null the offset error term to 1⁄4 LSB of full-scale for

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.

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

ADC0803 数据表(PDF) 34 Page - National Semiconductor (TI)