
AD7851
–16–
REV. A
4.096 V/16384 = 0.25 mV
when
V
REF
= 4.096 V. T he ideal in-
put/output transfer characteristic for the unipolar range is shown
in Figure 16.
+FS –1LSB
OUTPUT
CODE
0V
111...111
111...110
111...101
111...100
000...011
000...001
000...000
000...010
V
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
1LSB
1LSB =
FS
16384
Figure 16. AD7851 Unipolar Transfer Characteristic
Figure 15 shows the AD7851’s
±
V
REF
/2 bipolar analog input
configuration (where AIN(+) cannot go below 0 V so for the full
bipolar range then the AIN(–) pin should be biased to +V
REF
/2).
Once again the designed code transitions occur midway between
successive integer LSB values. T he output coding is 2s comple-
ment with
1 LSB = 16384 = 4.096 V/16384 = 0.25 mV.
T he
ideal input/output transfer characteristic is shown in
Figure 17.
– 1 LSB
FS = V
REF
V
1LSB =
FS
16384
OUTPUT
CODE
011...111
V
REF
/2
011...110
000...001
000...000
100...001
100...000
100...010
V
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
0V
+ FS
111...111
(V
REF
/2) –1 LSB
(V
REF
/2) +1 LSB
Figure 17. AD7851 Bipolar Transfer Characteristic
Input Ranges
T he analog input range for the AD7851 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(–) has to be biased up
to +V
REF
/2 and the output coding is 2s complement (See T able
V and Figures 14 and 15). T he unipolar or bipolar mode is se-
lected by the AMODE pin (0 for the unipolar range and 1 for
the bipolar range).
T able V. Analog Input Connections
Analog Input
Range
Input Connections Connection
AIN(+)
AIN(–)
Diagram
AMODE
0 V to V
REF1
±
V
REF
/2
2
V
IN
V
IN
AGND
V
REF
/2
Figure 8
Figure 9
DGND
DV
DD
NOT ES
1
Output code format is straight binary.
2
Range is
±
V
REF
/2 biased about V
REF
/2. Output code format is 2s complement.
Note that the AIN(–) pin on the AD7851 can be biased up
above AGND in the unipolar mode also, if required. T he ad-
vantage of biasing the lower end of the analog input range away
from AGND is that the user does not have to have the analog
input swing all the way down to AGND. T his has the advantage
in true single supply applications that 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(–) must be
tied to AGND in unipolar mode.
AIN(+)
AIN(–)
AMODE
AD7851
UNIPOLAR
ANALOG
INPUT RANGE
SELECTED
DOUT
STRAIGHT
BINARY
FORMAT
V
IN
= 0 TO V
REF
TRACK AND HOLD
AMPLIFIER
Figure 14. 0 V to V
REF
Unipolar Input Configuration
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/16384 =
2S
COMPLEMENT
FORMAT
V
REF
/2
DV
DD
AIN(+)
AIN(–)
AMODE
AD7851
UNIPOLAR
ANALOG
INPUT RANGE
SELECTED
DOUT
V
IN
= 0 TO V
REF
TRACK AND HOLD
AMPLIFIER
Figure 15.
±
V
REF
/2 about V
REF
/2 Bipolar Input Configuration