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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 VDD and VSS supply

must be equal to or greater than the maximum analog input

range selected (see Table 6).

VDRIVE

The AD7321 has a VDRIVE feature to control the voltage at which

the serial interface operates. VDRIVE allows the ADC to easily

interface to both 3 V and 5 V processors. For example, if the

AD7321 is operated with a VCC of 5 V, the VDRIVE 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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