Datasheet

Data Sheet AD7321
Rev. B | Page 27 of 36
REFERENCE
The AD7321 can operate with either the internal 2.5 V on-chip
reference or an externally applied reference. The internal reference
is selected by setting the Ref bit in the control register to 1. On
power-up, the Ref bit is 0, which selects the external reference
for the AD7321 conversion. Suitable reference sources for the
AD7321 include AD780, AD1582, ADR431, REF193, and
ADR391.
The internal reference circuitry consists of a 2.5 V band gap
reference and a reference buffer. When operating the AD7321
in internal reference mode, the 2.5 V internal reference is available
at the REFIN/OUT pin, which should be decoupled to AGND
using a 680 nF capacitor. It is recommended that the internal
reference be buffered before applying it elsewhere in the system.
The internal reference is capable of sourcing up to 90 μA.
On power-up, if the internal reference operation is required for
the ADC conversion, a write to the control register is necessary
to set the Ref bit to 1. During the control register write, the
conversion result from the first initial conversion is invalid. The
reference buffer requires 500 µs to power up and charge the
680 nF decoupling capacitor during the power-up time.
The AD7321 is specified for a 2.5 V to 3 V reference range.
When a 3 V reference is selected, the ranges are ±12 V, ±6 V,
±3 V, and 0 V to +12 V. For these ranges, the V
DD
and V
SS
supply
must be equal to or greater than the maximum analog input
range selected (see Table 6).
V
DRIVE
The AD7321 has a V
DRIVE
feature to control the voltage at which
the serial interface operates. V
DRIVE
allows the ADC to easily
interface to both 3 V and 5 V processors. For example, if the
AD7321 is operated with a V
CC
o f 5 V, t h e V
DRIVE
pin can be
powered from a 3 V supply. This allows the AD7321 to accept
large bipolar input signals with low voltage digital processing.
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