Alternatively, an external reference can be applied to the VRE" />
參數(shù)資料
型號： EVAL-AD7938CBZ
廠商： Analog Devices Inc
文件頁數(shù)： 17/36頁
文件大?。?/td> 0K
描述： EVAL BOARD FOR AD7938
標準包裝： 1
ADC 的數(shù)量： 1
位數(shù)： 12
采樣率（每秒）： 1.5M
數(shù)據(jù)接口： 并聯(lián)
輸入范圍： ±VREF
在以下條件下的電源（標準）： 13.5mW @ 5V
工作溫度： -40°C ~ 85°C
已用 IC / 零件： AD7938
已供物品： 板，CD
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其它名稱： Q3627458
AD7938/AD7939
Data Sheet
Rev. C | Page 24 of 36
Alternatively, an external reference can be applied to the VREFIN/
VREFOUT 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 VDD.
It is important to ensure that, when choosing the reference
value, the maximum analog input range (VINMAX) is never
greater than VDD + 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 VDD, the 0 V to 2 × VREF range
cannot be used. This is because the analog input signal range
now extends to 2 × VDD, which exceeds the maximum rating
conditions. In the pseudo differential modes, the user must
ensure that VREF + (VIN) ≤ VDD when using the 0 V to VREF range,
or when using the 2 × VREF range that 2 × VREF + (VIN) ≤ VDD.
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 (A)
2.5/3
0.04
1000
2.5
0.04
500
2.048
0.05
500
03715-
029
1
AD780
NC
8
2
+VIN
NC
7
3
GND
6
4
TEMP
5
O/P SELECT
TRIM
VOUT
VREF
2.5V
NC
VDD
NC = NO CONNECT
10nF
0.1F
*ADDITIONAL PINS OMITTED FOR CLARITY.
AD7938/
AD7939*
Figure 35. Typical VREF 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 VDD + 0.3 V limit as on the analog inputs.
Another advantage of the digital inputs not being restricted by
the VDD + 0.3 V limit is the fact that power supply sequencing
issues are avoided. If any of these inputs are applied before VDD,
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 VDD.
VDRIVE Input
The AD7938/AD7939 have a VDRIVE feature. VDRIVE controls the
voltage at which the parallel interface operates. VDRIVE allows the
ADC to easily interface to 3 V and 5 V processors.
For example, if the AD7938/AD7939 are operated with an VDD
of 5 V and the VDRIVE pin is powered from a 3 V supply, the
AD7938/AD7939 have better dynamic performance with an
VDD of 5 V while still being able to interface directly to 3 V
processors. Care should be taken to ensure VDRIVE does not
exceed VDD by more than 0.3 V (see the Absolute Maximum
Ratings section).
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