Datasheet

AD7938-6 Data Sheet
Rev. C | Page 22 of 32
e d r en e is 2.5 V.
eference values is
f suitable voltage references from
Table 12. Examples of Suitable Voltage References
Reference
ting
)
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 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 sp cifie efer c
The performance of the part with different r
shown in Figure 9 to Figure 11. 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
AD7938-6 transfer function and add to specified full-scale
errors on the part.
Table 12 lists examples o
Analog Devices that can be used. Figure 34 shows a typical
connection diagram for an external reference.
Output Initial Accuracy Opera
Voltage (% Maximum) Current (μA
AD780 2.5/3 0.04 1000
ADR421 2.5 0.04 500
ADR420 8 2.04 0.05 500
Digital Inputs
The digital inputs applied to the AD7938-6 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 were applied prior to V
DD.
V
DRIVE
Input
The AD7938-6 has 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 AD7938-6 is operated with an AV
DD
of 5 V
and the V
DRIVE
pin is powered from a 3 V supply, the AD7938-6
has better dynamic performance with a 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).
AD7938-6*
1
AD780
NC
8
04751-042
2
+V
IN
NC
7
3
GND
6
4
TEMP
5
V
REF
O/PSELECT
TRIM
V
OUT
NC
2.5V
NC
V
DD
NC = NO CONNECT
10nF 0.1µF
0.1µF
0.1µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 34. Typical V
REF
Connection Diagram