Datasheet

AD7938/AD7939 Data Sheet
Rev. C | Page 24 of 36
Alternatively, an external reference can be applied to the V
REFIN
/
V
REFOUT
pin of the AD7938/AD7939. An external reference
input is selected by setting the REF bit in the internal control
register to 0. The external reference input range is 0.1 V to V
DD
.
It is important to ensure that, when choosing the reference
value, the maximum analog input range (V
IN MAX
) is never
greater than V
DD
+ 0.3 V, to comply with the maximum ratings
of the device. For example, if operating in differential mode and
the reference is sourced from V
DD
, the 0 V to 2 × V
REF
range
cannot be used. This is because the analog input signal range
now extends to 2 × V
DD
, which exceeds the maximum rating
conditions. In the pseudo differential modes, the user must
ensure that V
REF
+ (V
IN−
) ≤ V
DD
when using the 0 V to V
REF
range,
or when using the 2 × V
REF
range that 2 × V
REF
+ (V
IN−
) ≤ V
DD
.
In all cases, the specified reference is 2.5 V.
The performance of the part with different reference values is
shown in Figure 10 to Figure 12. The value of the reference sets
the analog input span and the common-mode voltage range.
Errors in the reference source result in gain errors in the
AD7938/AD7939 transfer function and add to specified full-
scale errors on the part.
Table 13 lists examples of suitable voltage references available
from Analog Devices that can be used. Figure 35 shows a typical
connection diagram for an external reference.
Table 13. Examples of Suitable Voltage References
Reference
Output
Voltage (V)
Initial Accuracy
(% Max)
Operating
Current (µA)
AD780 2.5/3 0.04 1000
ADR421 2.5 0.04 500
ADR420 2.048 0.05 500
03715-029
1
AD780
NC
8
2
+V
IN
NC
7
3
GND
6
4
TEMP
5
O/P SELECT
TRIM
V
OUT
V
REF
2.5V
NC
NC
V
DD
NC = NO CONNECT
10nF 0.1µF
0.1µF
0.1µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
AD7938/
AD7939*
Figure 35. Typical V
REF
Connection Diagram
Digital Inputs
The digital inputs applied to the AD7938/AD7939 are not
limited by the maximum ratings that limit the analog inputs.
Instead, the digital inputs applied can go to 7 V and are not
restricted by the V
DD
+ 0.3 V limit as on the analog inputs.
Another advantage of the digital inputs not being restricted by
the V
DD
+ 0.3 V limit is the fact that power supply sequencing
issues are avoided. If any of these inputs are 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 was applied prior to V
DD.
V
DRIVE
Input
The AD7938/AD7939 have a V
DRIVE
feature. V
DRIVE
controls the
voltage at which the parallel interface operates. V
DRIVE
allows the
ADC to easily interface to 3 V and 5 V processors.
For example, if the AD7938/AD7939 are operated with an V
DD
of 5 V and the V
DRIVE
pin is powered from a 3 V supply, the
AD7938/AD7939 have better dynamic performance with an
V
DD
of 5 V while still being able to interface directly to 3 V
processors. Care should be taken to ensure V
DRIVE
does not
exceed V
DD
by more than 0.3 V (see the Absolute Maximum
Ratings section).