
AD7854/AD7854L
REV. 0
–15–
T ransfer Functions
For the unipolar range the designed code transitions occur mid-
way between successive integer LSB values (i.e., 1/2 LSB,
3/2 LSBs, 5/2 LSBs . . . FS – 3/2 LSBs). T he output coding is
straight binary for the unipolar range with 1
LSB = FS
/4096
=
3.3
V/
4096
=
0.8
mV
when
V
REF
= 3.3
V
. T he ideal input/
output transfer characteristic for the unipolar range is shown in
Figure 14.
OUTPUT
CODE
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
0V 1LSB
V
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
+FS –1LSB
1LSB =
FS
4096
Figure 14. AD7854/AD7854L Unipolar Transfer
Characteristic
Figure 13 shows the AD7854/AD7854L’s
±
V
REF
/2 bipolar ana-
log input configuration. AIN(+) cannot go below 0 V, so for the
full bipolar range, AIN(–) should be biased to at least +V
REF
/2.
Once again the designed code transitions occur midway between
successive integer L SB values. T he output coding is twos
complement with 1
LSB =
4096
=
3.3
V
/4096
=
0.8
mV
. T he
ideal input/output transfer characteristic is shown in Figure 15.
OUTPUT
CODE
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
V
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
1LSB =
FS
4096
0V
(V
REF
/2) – 1LSB
(V
REF
/2) + 1 LSB
V
REF
/2
+ FS – 1LSB
FS = V
REF
V
Figure 15. AD7854/AD7854L Bipolar Transfer Characteristic
Input Ranges
T he analog input range for the AD7854/AD7854L is 0 V to
V
REF
in both the unipolar and bipolar ranges.
T he only difference between the unipolar range and the bipolar
range is that in the bipolar range the AIN(–) should be biased
up to at least +V
REF
/2 and the output coding is twos comple-
ment (see T able V and Figures 14 and 15).
T able V. Analog Input Connections
Analog Input
Range
Input Connections
AIN(+)
Connection
Diagram
AIN(–)
0 V to V
REF1
±
V
REF
/2
2
V
IN
V
IN
AGND
V
REF
/2
Figure 12
Figure 13
NOT ES
1
Output code format is straight binary.
2
Range is
±
V
REF
/2 biased about V
REF
/2. Output code format is twos complement.
Note that the AIN(–) pin on the AD7854/AD7854L can be bi-
ased up above AGND in the unipolar mode, or above V
REF
/2 in
bipolar mode if required. T he advantage of biasing the lower
end of the analog input range away from AGND is that the ana-
log input does not have to swing all the way down to AGND.
T hus, in single supply applications the input amplifier does not
have to swing all the way down to AGND. T he upper end of the
analog input range is shifted up by the same amount. Care must
be taken so that the bias applied does not shift the upper end of
the analog input above the AV
DD
supply. In the case where the
reference is the supply, AV
DD
, the AIN(–) should be tied to
AGND in unipolar mode or to AV
DD
/2 in bipolar mode.
. . .
AIN(+)
AIN(–)
V
IN
= 0 TO V
REF
TRACK AND HOLD
AMPLIFIER
DB0
DB11
STRAIGHT
BINARY
FORMAT
AD7854/AD7854L
Figure 12. 0 to V
REF
Unipolar Input Configuration
2’S
COMPLEMENT
FORMAT
V
REF
/2
. . .
AIN(+)
AIN(–)
V
IN
= 0 TO V
REF
TRACK AND HOLD
AMPLIFIER
DB0
DB11
AD7854/AD7854L
Figure 13.
±
V
REF
/2 about V
REF
/2 Bipolar Input Configuration