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AD7298

Rev. B | Page 15 of 24

Temperature Value Format

VDRIVE

One LSB of the ADC corresponds to 0.25°C. The temperature

reading from the ADC is stored in a 12-bit twos complement

format to accommodate both positive and negative temperature

measurements. The temperature data format is provided in

Table 6.

The AD7298 also provides the VDRIVE feature. VDRIVE controls the

voltage at which the serial interface operates. VDRIVE allows the

ADC to easily interface to both 1.8 V and 3 V processors. For

example, if the AD7298 is operated with a VDD of 3.3 V, the

VDRIVE pin can be powered from a 1.8 V supply.

This enables the AD7298 to operate with a larger dynamic

range with a VDD of 3.3 V while still being able to interface to

1.8 V processors. Take care to ensure VDRIVE does not exceed

VDD by more than 0.3 V (see the Absolute Maximum Ratings

section).

Table 6. Temperature Data Format

Temperature (°C)

Digital Output

−40

1111 0110 0000

−25

1111 1001 1100

−10

1111 1101 1000

THE INTERNAL OR EXTERNAL REFERENCE

−0.25

1111 1111 1111

0000 0000 0000

The AD7298 can operate with either the internal 2.5 V on-chip

reference or an externally applied reference. The EXT_REF bit

in the control register is used to determine whether the internal

reference is used. If the EXT_REF bit is selected in the control

VREF pin. On power-up, the internal reference is enabled.

Suitable external reference sources for the AD7298 include

AD780, AD1582, ADR431, REF193, and ADR391.

+0.25

0000 0000 0001

+10

0000 0010 1000

+25

0000 0110 0100

+50

0000 1100 1000

+75

0001 0010 1100

+100

0001 1001 0000

+105

0001 1010 0100

+125

0001 1111 0100

The internal reference circuitry consists of a 2.5 V band gap

reference and a reference buffer. When the AD7298 operates in

internal reference mode, the 2.5 V internal reference is available

at the VREF pin, which should be decoupled to GND1 using a 10 μF

capacitor. It is recommended that the internal reference be buffered

before applying it elsewhere in the system.

The temperature conversion formulas are as follows:

Positive Temperature = ADC Code/4

Negative Temperature = (4096 − ADC Code)/4

The previous formulas are for a VREF of 2.5 V only.

If an external reference is used, the temperature sensor requires

an external reference of between 2 V and 2.5 V for correct

operation. When an external reference of less than 2.5 V is

applied, the temperature results are calculated using the

following formula, where VEXT_REF is the value of the external

The internal reference is capable of sourcing up to 2 mA of

current when the converter is static. The reference buffer

requires 5.5 ms to power up and charge the 10 μF decoupling

capacitor during the power-up time.

reference voltage.

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ADCCode

Temperatur

REF

EXT

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