EVAL-AD7321CB【PDF 27/36 页】500 kSPS, 2-Channel, Software-Selectable

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AD7321

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.

Rev. 0 | Page 27 of 36

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

and V

supply

must be equal to or greater than the maximum analog input

range selected (see Table 6).

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

of 5 V, the 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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