Datasheet

Data Sheet AD7904/AD7914/AD7924
Rev. C | Page 19 of 32
ADC TRANSFER FUNCTION
The output coding of the AD7904/AD7914/AD7924 is either
straight binary or twos complement, depending on the status of
the LSB in the control register. The designed code transitions
occur at successive LSB values (that is, 1 LSB, 2 LSBs, and so on).
For the 0 V to REF
IN
input range, the LSB size is REF
IN
/256 for
the AD7904, REF
IN
/1024 for the AD7914, and REF
IN
/4096 for
the AD7924. For the 0 V to 2 × REF
IN
input range, the LSB size
is 2 × REF
IN
/256 for the AD7904, 2 × REF
IN
/1024 for the AD7914,
and 2 × REF
IN
/4096 for the AD7924. The ideal transfer charac-
teristic for the AD7904/AD7914/AD7924 when straight binary
coding is selected is shown in Figure 16; the ideal transfer
characteristic for the AD7904/AD7914/AD7924 when twos
complement coding is selected is shown in Figure 17.
ADC CODE
000…000
0V
ANALOG INPUT
111…111
000…001
000…010
111…110
•
•
111…000
•
011…111
•
•
1LSB
+V
REF
– 1LSB
NOTES
1. V
REF
IS EITHER REF
IN
OR 2 × REF
IN
.
1LSB = V
REF
/256 AD7904
1LSB = V
REF
/1024 AD7914
1LSB = V
REF
/4096 AD7924
03087-016
Figure 16. Straight Binary Transfer Characteristic
1LSB = 2 × V
REF
/256 AD7904
1LSB = 2 × V
REF
/1024 AD7914
1LSB = 2 × V
REF
/4096 AD7924
ADC CODE
ANALOG INPUT
100…000
011…111
100…001
100…010
011…110
•
•
000…001
111…111
•
•
000…000
–V
REF
+ 1LSB +V
REF
– 1LSB
V
REF
– 1LSB
0
3087-017
Figure 17. Twos Complement Transfer Characteristic
with 0 V to 2 × REF
IN
Input Range
Handling Bipolar Input Signals
Figure 18 shows how the combination of the 0 V to 2 × REF
IN
input range and the twos complement output coding scheme is
particularly useful for handling bipolar input signals. If the
bipolar input signal is biased about REF
IN
and twos complement
output coding is selected, 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
V
IN
0
REF
IN
V
IN
3
V
AV
DD
DOUT
REF
IN
+REF
IN
–REF
IN
011…111
000…000
100…000
0
V
V
R1
R1 = R2 = R3 = R4
V
DD
V
DRIVE
AD7904/
AD7914/
AD7924
V
REF
0.1µF
TWOS
COMPLEMENT
(= 0V)
(= 2 × REF
IN
)
V
DD
DSP/
MICRO-
PROCESSOR
03087-018
Figure 18. Handling Bipolar Signals