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Data Sheet

AD7609

Rev. A | Page 23 of 36

INTERNAL/EXTERNAL REFERENCE

The AD7609 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 AD7609. An

externally applied reference of 2.5 V is also amplified 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 and the external reference.

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

and is enabled; if this pin is set to logic low, the internal refer-

ence 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 AD7609 operates in

the reference mode selected by the REF SELECT pin. Decoupling

is required on the REFIN/REFOUT pin for both the internal

or external reference options. A 10 µF ceramic capacitor is

required on the REFIN/REFOUT to ground close to the

REFGND pins. The AD7609 contains a reference buffer

configured to amplify the REF voltage up to ~4.5 V, as shown

in Figure 38. The REFCAPA and REFCAPB pins must be

shorted together externally and a ceramic capacitor of 10 μF

applied to REFGND to ensure the reference buffer is in

closed-loop operation. The reference voltage available at the

REFIN/REFOUT pin is 2.5 V.

When the AD7609 is configured in external reference mode,

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

applications using multiple AD7609 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 AD7609 devices (see Figure 39). In

this configuration, each AD7609 REFIN/REFOUT pin should

be decoupled with a 100 nF decoupling capacitor.

Internal Reference Mode

One AD7609 device, configured to operate in the internal

reference mode, can be used to drive the remaining AD7609

devices, which are configured to operate in external reference

mode (see Figure 40). The REFIN/REFOUT pin of the AD7609,

configured in internal reference mode, should be decoupled

using a 10 µF ceramic decoupling capacitor. The other AD7609

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

REFCAPA

10µF

Figure 38. Reference Circuitry

AD7609

REF SELECT

REFIN/REFOUT

10µF

AD7609

REF SELECT

REFIN/REFOUT

100nF

AD7609

REF SELECT

REFIN/REFOUT

100nF

VDRIVE

Figure 39. Single External Reference Driving Multiple AD7609

REFIN/REFOUT Pins

AD7609

REF SELECT

REFIN/REFOUT

AD7609

REF SELECT

REFIN/REFOUT

100nF

0.1µF

100nF

AD7609

REF SELECT

REFIN/REFOUT

100nF

ADR421

Figure 40. Internal Reference Driving Multiple AD7609 REFIN Pins

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