Datasheet

AD7266
Rev. B | Page 18 of 28
TRANSFER FUNCTIONS
The designed code transitions occur at successive integer LSB
values (1 LSB, 2 LSB, and so on). In single-ended mode, the LSB
size is V
REF
/4096 when the 0 V to V
REF
range is used, and the
LSB size is 2 × V
REF
/4096 when the 0 V to 2 × V
REF
range is used.
In differential mode, the LSB size is 2 × V
REF
/4096 when the 0 V
to V
REF
range is used, and the LSB size is 4 × V
REF
/4096 when the
0 V to 2 × V
REF
range is used. The ideal transfer characteristic
for the AD7266 when straight binary coding is output is shown
in Figure 32, and the ideal transfer characteristic for the
AD7266 when twos complement coding is output is shown in
Figure 33 (this is shown with the 2 × V
REF
range).
04603-027
000...000
111...111
1LSB = V
REF
/4096
1LSB
V
REF
– 1LSB
ANALOG INPUT
ADC CODE
0V
000...001
000...010
111...110
111...000
011...111
NOTE
1. V
REF
IS EITHER V
REF
OR 2 × V
REF
.
Figure 32. Straight Binary Transfer Characteristic
04603-028
100...000
011...111
1LSB = 2
× V
REF
/4096
+V
REF
– 1 LSB–V
REF
+ 1LSB V
REF
– 1LSB
ANALOG INPUT
ADC CODE
100...001
100...010
011...110
000...001
000...000
111...111
Figure 33. Twos Complement Transfer Characteristic with
V
REF
± V
REF
Input Range
DIGITAL INPUTS
The digital inputs applied to the AD7266 are not limited by the
maximum ratings that limit the analog inputs. Instead, the
digital inputs can be applied up to 7 V and are not restricted by
the V
DD
+ 0.3 V limit as are the analog inputs. See the Absolute
Maximum Ratings section for more information. Another
advantage of the SCLK, RANGE, A0 to A2, and
CS
pins not
being restricted by the V
DD
+ 0.3 V limit is that power supply
sequencing issues are avoided. If one of these digital inputs is
applied before V
DD
, there is no risk of latch-up, as there would
be on the analog inputs if a signal greater than 0.3 V were
applied prior to V
DD
.
V
DRIVE
The AD7266 also has a V
DRIVE
feature to control the voltage at
which the serial interface operates. V
DRIVE
allows the ADC to
easily interface to both 3 V and 5 V processors. For example, if
the AD7266 was operated with a V
DD
of 5 V, the V
DRIVE
pin
could be powered from a 3 V supply, allowing a large dynamic
range with low voltage digital processors. Therefore, the
AD7266 could be used with the 2 × V
REF
input range, with a V
DD
of 5 V while still being able to interface to 3 V digital parts.