AD7606/AD7606-6/AD7606-4
Internal Reference Mode
INTERNAꢀ/EXTERNAꢀ REFERENCE
One AD7606/AD7606-6/AD7606-4 device, configured to operate
in the internal reference mode, can be used to drive the remaining
AD7606/AD7606-6/AD7606-4 devices, which are configured to
operate in external reference mode (see Figure 42). The REFIN/
REFOUT pin of the AD7606/AD7606-6/AD7606-4, configured
in internal reference mode, should be decoupled using a 10 μF
ceramic decoupling capacitor. The other AD7606/AD7606-6/
AD7606-4 devices, configured in external reference mode,
should use at least a 100 nF decoupling capacitor on their
REFIN/REFOUT pins.
The AD7606/AD7606-6/AD7606-4 contain 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 AD7606/AD7606-6/AD7606-4. 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 AD7606/AD7606-6/AD7606-4
operate 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.
REFIN/REFOUT
SAR
REFCAPA
BUF
10µF
REFCAPB
2.5V
REF
Figure 40. Reference Circuitry
The AD7606/AD7606-6/AD7606-4 contain a reference buffer
configured to gain the REF voltage up to ~4.5 V, as shown in
Figure 40. The REFCAPA and REFCAPB pins must be shorted
together externally, and a ceramic capacitor of 10 ꢀF 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.
AD7606
AD7606
AD7606
REF SELECT
REF SELECT
REF SELECT
REFIN/REFOUT
REFIN/REFOUT
REFIN/REFOUT
100nF
100nF
100nF
ADR421
0.1µF
When the AD7606/AD7606-6/AD7606-4 are configured in
external reference mode, the REFIN/REFOUT pin is a high
input impedance pin. For applications using multiple AD7606
devices, the following configurations are recommended,
depending on the application requirements.
Figure 41. Single External Reference Driving Multiple AD7606/AD7606-6/
AD7606-4 REFIN Pins
V
DRIVE
External Reference Mode
AD7606
AD7606
AD7606
REF SELECT
REF SELECT
REF SELECT
One ADR421 external reference can be used to drive the
REFIN/REFOUT pins of all AD7606 devices (see Figure 41).
In this configuration, each REFIN/REFOUT pin of the
AD7606/AD7606-6/AD7606-4 should be decoupled with at
least a 100 nF decoupling capacitor.
REFIN/REFOUT
REFIN/REFOUT
REFIN/REFOUT
+
10µF
100nF
100nF
Figure 42. Internal Reference Driving Multiple AD7606/AD7606-6/AD7606-4
REFIN Pins
Rev. 0 | Page 24 of 36
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