EVAL-AD7328CB【PDF 27/36 页】8-Channel, Software-Selectable True Bipo

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AD7328

REFERENCE

The AD7328 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 AD7328 conversion. Suitable reference sources for the

AD7328 include

AD780

AD1582

ADR431

REF193

, and

ADR391

Rev. A | Page 27 of 36

The internal reference circuitry consists of a 2.5 V band gap

reference and a reference buffer. When operating the AD7328

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 con-

version 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 AD7328 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.

DRIVE

The AD7328 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

AD7328 is operated with a V

of 5 V, the V

DRIVE

pin can be

powered from a 3 V supply. This allows the AD7328 to accept

large bipolar input signals with low voltage digital processing.

TEMPERATURE INDICATOR

The AD7328 has an on-chip temperature indicator. The

temperature indicator can be used to provide local temperature

measurements on the AD7328. To access the temperature

indicator, the ADC should be configured in pseudo differential

mode, Mode 1 = Mode 0 = 1, which sets Channel Bits ADD2,

ADD1, and ADD0 to 1. V

7 must be tied to AGND or to a

small dc voltage within the specified pseudo input range for the

selected analog input range. When a conversion is initiated in

this configuration, the output code represents the temperature

(see Figure 46 and Figure 47). When using the temperature

indicator on the AD7328, the part should be operated at low

throughput rates, such as approximately 50 kSPS for the ±10 V

range and 30 kSPS for the ±2.5 V range. The throughput rate is

reduced for the temperature indicator mode because the AD7328

requires more acquisition time for this mode.

4420

4340

–40

100

TEMPERATURE (°C)

4410

4400

4390

4380

4370

4360

4350

–20

±10V RANGE, INT REF

= V

DRIVE

= 5V

50kSPS

Figure 46. Temperature vs. ADC Output Code for ±10 V Range

5450

5100

–40

TEMPERATURE (°C)

5400

5350

5300

5250

5200

5150

–20

= V

DRIVE

= 5V

±2.5V RANGE

INT REFERENCE

30kSPS

Figure 47. Temperature vs. ADC Output Code for ±2.5 V Range

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