REV. B
–16–
AD7853/AD7853L
AIN(+)
AIN(–)
AMODE
AD7853/AD7853L
BIPOLAR
ANALOG
INPUT RANGE
SELECTED
DOUT
2S
COMPLEMENT
FORMAT
V
IN
= 0 TO V
REF
TRACK AND HOLD
AMPLIFIER
V
REF
/2
DV
DD
Figure 15.
±
V
REF
/2 about V
REF
/2 Bipolar Input Configuration
+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
4096
Figure 16. Unipolar Transfer Characteristic
Figure 15 shows the AD7853/AD7853L’s ±V
REF
/2 bipolar ana-
log 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. The output
coding is twos complement with 1 LSB = 4096 = 3.3 V/4096 =
0.8 mV. The ideal input/output transfer characteristic is shown
in Figure 17.
– 1 LSB
FS = V
REF
V
1LSB =
FS
4096
OUTPUT
CODE
V
REF
/2
011...111
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. Bipolar Transfer Characteristic
IC1
AD820
AD820-3V
0.1mF
10mF
V+
V–
10kV
50V
10nF
(NPO)
TO AIN(+) OF
AD7853/AD7853L
V
IN
–V
REF
/2 TO +V
REF
/2
V
REF
/2
10kV
10kV
10kV
+3V TO +5V
Figure 13. Analog Input Buffering
Input Ranges
The analog input range for the AD7853/AD7853L is 0 V to
V
REF
in both the unipolar and bipolar ranges.
The 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 twos complement (See
Table V and Figures 14 and 15). The unipolar or bipolar mode
is selected by the AMODE pin (0 for the unipolar range and 1
for the bipolar range).
Table V. Analog Input Connections
Analog Input Input Connections Connection
Range AIN(+) AIN(–) Diagram AMODE
0 V to V
REF
1
V
IN
AGND Figure 14 DGND
±V
REF
/2
2
V
IN
V
REF
/2 Figure 15 DV
DD
NOTES
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 AD7853/AD7853L can be
biased up above AGND in the unipolar mode also, if required.
The advantage 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. This has the
advantage in true single supply applications that the input am-
plifier does not have to swing all the way down to AGND. The
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
AD7853/AD7853L
UNIPOLAR ANALOG
INPUT RANGE
SELECTED
DOUT
STRAIGHT
BINARY
FORMAT
V
IN
= 0 TO V
REF
TRACK AND HOLD
AMPLIFIER
Figure 14. 0 to V
REF
Unipolar Input Configuration
Transfer Functions
For the unipolar range the designed code transitions occur
midway between successive integer LSB values (i.e., 1/2 LSB,
3/2 LSBs, 5/2 LSBs . . . FS –3/2 LSBs). The 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. The ideal input/output
transfer characteristic for the unipolar range is shown in
Figure 16.
