AD7923 datasheet(16/25 Pages) AD

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Data Sheet

AD7923

Rev. D | Page 15 of 24

ADC TRANSFER FUNCTION

The output coding of the AD7923 is either straight binary or

twos complement, depending on the status of the LSB in the

control register. The designed code transitions occur at succes-

sive LSB values (for example, 1 LSB, 2 LSBs). The LSB size is

REFIN /4096 for the AD7923. The ideal transfer characteristic

for the AD7923 when straight binary coding is selected is

shown in Figure 16 and the ideal transfer characteristic for the

AD7923 when twos complement coding is selected is shown in

Figure 17.

+VREF – 1LSB

000...000

ANALOG INPUT

111...111

000...001

000...010

111...110

•

111...000

•

011...111

•

1LSB

1LSB = VREF/4096

NOTES

1. VREF IS EITHER REFIN OR 2 × REFIN

Figure 16. Straight Binary Transfer Characteristic

–VREF + 1LSB

ANALOG INPUT

+VREF – 1LSB

1LSB = 2

× V

REF/4096

VREF – 1LSB

100...000

011...111

100...001

100...010

011...110

•

000...001

111...111

•

000...000

Figure 17. Twos Complement Transfer Characteristic

with REFIN ± REFIN Input Range

Handling Bipolar Input Signals

Figure 18 shows how useful the combination of the 2 × REFIN

input range and the twos complement output coding scheme is

for handling bipolar input signals. If the bipolar input signal is

biased around REFIN and twos complement output coding is

selected, then REFIN becomes the zero-code point, −REFIN is

negative full scale, and + REFIN becomes positive full scale with

a dynamic range of 2 × REFIN.

REFIN

VIN0

VIN3

AD7923

DSP/

µP

VDD

0.1

µF

AVDD

VDRIVE

DOUT

TWOS

COMPLEMENT

+REFIN

REFIN

–REFIN

011...111

000...000

100...000

(= 0V)

(= 2

× REF

IN)

R1 = R2 = R3 = R4

VDD

VREF

•

Figure 18. Handling Bipolar Signals

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