參數(shù)資料
型號(hào)： AD7938
廠商： ANALOG DEVICES INC
元件分類： ADC
英文描述： 8-Channel, 1.5 MSPS, 12-Bit and 10-Bit Parallel ADCs with a Sequencer
中文描述： 8-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, QCC32
封裝： MO-220-VHHD, LFCSP-32
文件頁(yè)數(shù)： 24/32頁(yè)
文件大小： 1332K
代理商： AD7938
AD7938/AD7939
Preliminary Technical Data
Therefore, when operating at
V
DD
= 5 V, the value of
V
REF
can
range from 100 mV to a maximum value of 3.5 V. When
V
DD
=
4.75 V,
V
REF
MAX
= 3.17 V.
Example 2
V
IN MAX
=
V
DD
+ 0.3
0
1
AD780
NC
8
2
+V
IN
NC
7
3
GND
6
4
TEMP
5
O/PSELECT
TRIM
V
OUT
V
REF
2.5V
NC
NC
V
DD
NC = NO CONNECT
10nF
0.1
μ
F
0.1
μ
F
0.1
μ
F
*ADDITIONAL PINS OMITTED FOR CLARITY
AD7938/
AD7939*
V
IN MAX
=
V
REF
+
V
REF
/2
If
V
DD
= 3.6 V, then
V
IN MAX
= 3.9 V.
Therefore, 3 ×
V
REF
/2 = 3.6 V.
V
REF
MAX
= 2.6 V
Therefore, when operating with at
V
DD
= 3 V, the value of
V
REF
can range from 100 mV to a maximum value of 2.4 V. When
V
DD
= 2.7 V,
V
REF
MAX
= 2 V.
These examples show that the maximum reference applied to the
AD7938/AD7939 is directly dependant on the value applied to V
DD
.
The performance of the part at different reference values is
shown in Figures TBD to TBD. 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 12
be used that are available from Analog Devices and F
shows a typical connection diagram for an external reference.
Table 12. Examples of Suitable Voltage References
Output
Voltage
AD780
2.5/3
ADR421
2.5
ADR420
2.048
lists examples of suitable voltage references that could
igure 37
Figure 37. 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 AV
DD
+ 0.3 V limit as on the analog inputs.
Another advantage of the digital inputs not being restricted by
the AV
DD
+ 0.3 V limit is the fact that power supply sequencing
issues are avoided. If any of these inputs are applied before
AV
DD
, then 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
AV
DD.
V
DRIVE
Input
The AD7938/AD7939 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.
Reference
Initial Accuracy
(% Max)
0.04
0.04
0.05
Operating
Current (μA)
1000
500
500
For example, if the AD7938/AD7939 operate with an AV
DD
of
5 V and the V
DRIVE
pin is powered from a 3 V supply, the
AD7938/AD7939 has better dynamic performance with an
AV
DD
of 5V while still being able to interface directly to 3 V
processors. Care should be taken to ensure V
DRIVE
does not
exceed AV
DD
by more than 0.3 V (see the
section).
Ratings
Absolute Maximum
Rev. PrN | Page 24 of 32
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