AD7939BCP datasheet(24/32 Pages) AD

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AD7938/AD7939

Preliminary Technical Data

Therefore, when operating at VDD = 5 V, the value of VREF can

range from 100 mV to a maximum value of 3.5 V. When VDD =

4.75 V, VREF MAX = 3.17 V.

AD780

+VIN

GND

TEMP

O/PSELECT

TRIM

VOUT

VREF

2.5V

VDD

NC = NO CONNECT

10nF

0.1

µF

0.1

µF

0.1

µF

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7938/

AD7939*

Example 2

VIN MAX = VDD + 0.3

VIN MAX = VREF + VREF/2

If VDD = 3.6 V, then VIN MAX = 3.9 V.

Therefore, 3 × VREF/2 = 3.6 V.

VREF MAX = 2.6 V

Therefore, when operating with at VDD = 3 V, the value of VREF

can range from 100 mV to a maximum value of 2.4 V. When

VDD = 2.7 V, VREF MAX = 2 V.

These examples show that the maximum reference applied to the

AD7938/AD7939 is directly dependant on the value applied to VDD.

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

Table 12. Examples of Suitable Voltage References

lists examples of suitable voltage references that could

be used that are available from Analog Devices and F

shows a typical connection diagram for an external reference.

igure 37

Figure 37. 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 AVDD + 0.3 V limit as on the analog inputs.

Another advantage of the digital inputs not being restricted by

the AVDD + 0.3 V limit is the fact that power supply sequencing

issues are avoided. If any of these inputs are applied before

AVDD, 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

AVDD.

VDRIVE Input

The AD7938/AD7939 has 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.

Reference

Output

Voltage

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

For example, if the AD7938/AD7939 operate with an AVDD of

5 V and the VDRIVE pin is powered from a 3 V supply, the

AD7938/AD7939 has better dynamic performance with an

AVDD of 5V while still being able to interface directly to 3 V

processors. Care should be taken to ensure VDRIVE does not

exceed AVDD by more than 0.3 V (see the

section).

Absolute Maximum

Ratings

Rev. PrN | Page 24 of 32

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