AD7938 Analog Devices, AD7938 Datasheet - Page 24

• Manufacturer Part Number: AD7938
• Description: 8-Channel, 1.5 MSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 1.5MSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,5V p-p,Uni (Vref),Uni (Vref) x 2,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7938/AD7939
Alternatively, an external reference can be applied to the V
V
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
It is important to ensure that, when choosing the reference
value, the maximum analog input range (V
greater than V
of the device. For example, if operating in differential mode and
the reference is sourced from V
cannot be used. This is because the analog input signal range
now extends to 2 × V
conditions. In the pseudo differential modes, the user must
ensure that V
or when using the 2 × V
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
AD780
ADR421
ADR420
V
*ADDITIONAL PINS OMITTED FOR CLARITY.
DD
0.1µF
REFOUT
pin of the AD7938/AD7939. An external reference
10nF
REF
Output
Voltage (V)
2.5/3
2.5
2.048
Figure 35. Typical V
DD
+ (V
0.1µF
+ 0.3 V, to comply with the maximum ratings
IN−
NC
DD
) ≤ V
, which exceeds the maximum rating
REF
NC = NO CONNECT
1
2
3
4
DD
range that 2 × V
+V
TEMP
GND
Initial Accuracy
(% Max)
0.04
0.04
0.05
O/P SELECT
IN
when using the 0 V to V
REF
AD780
DD
Connection Diagram
, the 0 V to 2 × V
V
TRIM
OUT
8
7
6
5
2.5V
IN MAX
REF
NC
NC
NC
+ (V
Operating
Current (µA)
1000
500
500
) is never
REF
IN−
REF
AD7938/
AD7939*
) ≤ V
range
0.1µF
range,
REFIN
DD
V
REF
Rev. C | Page 24 of 36
DD
.
/
.
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
Another advantage of the digital inputs not being restricted by
the V
issues are avoided. If any of these inputs are applied before V
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
V
The AD7938/AD7939 have a V
voltage at which the parallel interface operates. V
ADC to easily interface to 3 V and 5 V processors.
For example, if the AD7938/AD7939 are operated with an V
of 5 V and the V
AD7938/AD7939 have better dynamic performance with an
V
processors. Care should be taken to ensure V
exceed V
Ratings section).
DRIVE
DD
of 5 V while still being able to interface directly to 3 V
DD
Input
+ 0.3 V limit is the fact that power supply sequencing
DD
by more than 0.3 V (see the Absolute Maximum
DRIVE
DD
+ 0.3 V limit as on the analog inputs.
pin is powered from a 3 V supply, the
DRIVE
feature. V
DRIVE
DRIVE
Data Sheet
DRIVE
does not
controls the
allows the
DD.
DD
DD
,