AD7938BCPZ datasheet(24/36 Pages) AD

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AD7938/AD7939

Data Sheet

Rev. D | 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

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 (µA)

AD780

2.5/3

0.04

1000

ADR421

2.5

0.04

500

ADR420

2.048

0.05

500

AD780

+VIN

GND

TEMP

O/P SELECT

TRIM

VOUT

VREF

2.5V

VDD

NC = NO CONNECT

10nF

0.1µF

*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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