Datasheet

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
09760-035
Figure 38. Reference Circuitry
AD7609
REF SELECT
REFIN/REFOUT
+
10µF
AD7609
REF SELECT
REFIN/REFOUT
100nF
AD7609
REF SELECT
REFIN/REFOUT
100nF
V
DRIVE
09760-036
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
09760-037
Figure 40. Internal Reference Driving Multiple AD7609 REFIN Pins