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AD7329

Rev. 0 | Page 31 of 40

REFERENCE

The AD7329 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 AD7329 conversion. Suitable reference sources

for the AD7329 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 AD7329

in internal reference mode, the 2.5 V internal reference is

available at the REFIN/REFOUT 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 AD7329 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.

VDRIVE

The AD7329 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

AD7329 is operated with a VCC of 5 V, the VDRIVE pin can be

powered from a 3 V supply. This allows the AD7329 to accept

large bipolar input signals with low voltage digital processing.

TEMPERATURE INDICATOR

The AD7329 has an on-chip temperature indicator. The

temperature indicator can be used to provide local temperature

measurements on the AD7329. 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. VIN7 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 50). When using the temperature indicator on the

AD7329, the part should be operated at low throughput rates, such

as approximately 30 kSPS for the ±2.5 V range. The throughput

rate is reduced for the temperature indicator mode because the

AD7329 requires more acquisition time for this mode.

5450

5050

–40

TEMPERATURE (°C)

VCC = VDRIVE = 5V

VDD = 12V, VSS = –12V

±2.5V RANGE

INTERNAL REFERENCE

30kSPS

5400

5350

5300

5250

5200

5150

5100

–20

204060

Figure 50. Temperature vs. ADC Output Code for ±2.5 V Range

4420

4340

–40

100

TEMPERATURE (°C)

4410

4400

4390

4380

4370

4360

4350

–20

20406080

±10V RANGE, INT REF

VCC =VDRIVE =5V

VDD/VSS = ±12V

50kSPS

Figure 51. Temperature vs. ADC Output Code for ±10 V Range

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