Datasheet

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
REF
IN
/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.
+V
REF
– 1LSB
000...000
0V
ANALOG INPUT
111...111
000...001
000...010
111...110
•
•
111...000
•
011...111
•
•
1LSB
1LSB = V
REF
/4096
NOTES
1. V
REF
IS EITHERREF
IN
OR 2 × REF
IN
ADC CODE
03086-016
Figure 16. Straight Binary Transfer Characteristic
–V
REF
+1LSB
ADC CODE
ANALOG INPUT
+V
R
EF
–
1LSB
1LSB = 2
×
V
REF
/
4
096
V
REF
–
1LSB
100...000
011...111
100...001
100...010
011...110
•
•
000...001
111...111
•
•
000...000
03086-017
Figure 17. Twos Complement Transfer Characteristic
with REF
IN
± REF
IN
Input Range
Handling Bipolar Input Signals
Figure 18 shows how useful the combination of the 2 × REF
IN
input range and the twos complement output coding scheme is
for handling bipolar input signals. If the bipolar input signal is
biased around REF
IN
and twos complement output coding is
selected, then REF
IN
becomes the zero-code point, −REF
IN
is
negative full scale, and + REF
IN
becomes positive full scale with
a dynamic range of 2 × REF
IN
.
R3
R2
R4
REF
IN
V
IN
0
V
IN
3
AD7923
DSP/µP
V
DD
0.1µ
F
V
AV
DD
V
DRIVE
DOUT
TWOS
COMPLEMENT
+REF
IN
REF
IN
–REF
IN
011...111
000...000
100...000
(= 0V)
(= 2 × REF
IN
)
0V
V
R1
R1 = R2 = R3 = R4
V
DD
V
REF
•
•
03086-018
Figure 18. Handling Bipolar Signals