AD7608BSTZ datasheet(21/32 Pages) AD

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Data Sheet

AD7608

Rev. A | Page 21 of 32

INTERNAL/EXTERNAL REFERENCE

The AD7608 contains an on-chip 2.5 V band gap 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 AD7608. 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 AD7608 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 AD7608 contains a reference buffer configured to gain the

REF voltage up to ~4.5 V, as shown in Figure 37. 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 AD7608 is configured in external reference mode,

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

applications using multiple AD7608 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 AD7608 devices (see Figure 38).

In this configuration, each REFIN/REFOUT pin of the AD7608

should be decoupled with at least a 100 nF decoupling capacitor.

Internal Reference Mode

One AD7608 device, configured to operate in the internal refer-

ence mode, can be used to drive the remaining AD7608 devices,

which are configured to operate in external reference mode (see

Figure 39). The REFIN/REFOUT pin of the AD7608, configured

in internal reference mode, should be decoupled using a 10 µF

ceramic decoupling capacitor. The other AD7608 devices,

configured in external reference mode, should use at least a

100 nF decoupling capacitor on their REFIN/REFOUT pins.

BUF

SAR

2.5V

REF

REFCAPB

REFIN/REFOUT

REFCAPA

10µF

Figure 37. Reference Circuitry

AD7608

REF SELECT

REFIN/REFOUT

AD7608

REF SELECT

REFIN/REFOUT

100nF

0.1µF

100nF

AD7608

REF SELECT

REFIN/REFOUT

100nF

ADR421

Figure 38. Single External Reference Driving Multiple AD7608 REFIN Pins

AD7608

REF SELECT

REFIN/REFOUT

10µF

AD7608

REF SELECT

REFIN/REFOUT

100nF

AD7608

REF SELECT

REFIN/REFOUT

100nF

VDRIVE

Figure 39. Internal Reference Driving Multiple AD7608 REFIN Pins

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