AD7607 datasheet(21/32 Pages) AD

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Data Sheet

AD7607

Rev. B | Page 21 of 32

INTERNAL/EXTERNAL REFERENCE

The AD7607 contains an on-chip 2.5 V bandgap reference. The

REFIN/REFOUT pin allows access to the 2.5 V reference that

generates the on-chip 4.5 V reference internally, or it allows

an external reference of 2.5 V to be applied to the AD7607. An

externally applied reference of 2.5 V is also gained up to 4.5 V, using

the internal buffer. This 4.5 V buffered reference is the reference

used by the SAR ADC.

The REF SELECT pin is a logic input pin that allows the user to

select between the internal reference or an external reference.

If this pin is set to logic high, the internal reference is selected

and enabled. If this pin is set to logic low, the internal reference

is disabled and an external reference voltage must be applied

to the REFIN/REFOUT pin. The internal reference buffer is

always enabled. After a reset, the AD7607 operates in the reference

mode selected by the REF SELECT pin. Decoupling is required

on the REFIN/REFOUT pin for both the internal and external

reference options. A 10 μF ceramic capacitor is required on the

REFIN/REFOUT pin.

The AD7607 contains a reference buffer configured to gain the

REF voltage up to ~4.5 V, as shown in Figure 36. The REFCAPA

and REFCAPB pins must be shorted together externally, and

a ceramic capacitor of 10 μF applied to REFGND, to ensure that

the reference buffer is in closed-loop operation. The reference

voltage available at the REFIN/REFOUT pin is 2.5 V.

When the AD7607 is configured in external reference mode,

the REFIN/REFOUT pin is a high input impedance pin. For

applications using multiple AD7607 devices, the following

configurations are recommended, depending on the application

requirements.

External Reference Mode

One ADR421 external reference can be used to drive the

REFIN/REFOUT pins of all AD7607 devices (see Figure 37).

In this configuration, each REFIN/REFOUT pin of the AD7607

should be decoupled with a 100 nF decoupling capacitor.

Internal Reference Mode

One AD7607 device, configured to operate in the internal

reference mode, can be used to drive the remaining AD7607

devices, which are configured to operate in external reference

mode (see Figure 38). The REFIN/REFOUT pin of the AD7607,

configured in internal reference mode, should be decoupled

using a 10 μF ceramic decoupling capacitor. The other AD7607

devices, configured in external reference mode, should use a

100 nF decoupling capacitor on their REFIN/REFOUT pins.

BUF

SAR

2.5V

REF

REFCAPB

REFIN/REFOUT

REFCAPB

10µF

Figure 36. Reference Circuitry

AD7607

REF SELECT

REFIN/REFOUT

AD7607

REF SELECT

REFIN/REFOUT

100nF

0.1µF

100nF

AD7607

REF SELECT

REFIN/REFOUT

100nF

ADR421

Figure 37. Single External Reference Driving Multiple AD7607 REFIN Pins

AD7607

REF SELECT

REFIN/REFOUT

10µF

AD7607

REF SELECT

REFIN/REFOUT

100nF

AD7607

REF SELECT

REFIN/REFOUT

100nF

VDRIVE

Figure 38. Internal Reference Driving Multiple AD7607 REFIN Pins.

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