AD7709 datasheet(26/32 Pages) AD

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REV. A

AD7709

–26–

A second key advantage to using the AD7709 in transducer-based

applications is that the on-chip low-side power switch can be fully

utilized in low power applications. The low-side power switch is

connected in series with the cold side of the bridge. In normal

operation, the switch is closed and measurements can be taken

from the bridge. In applications where power is a concern, the

AD7709 can be put into low power mode, substantially reducing

the power burned in the application. In addition to this, the power

switch can be opened while in low power mode, thus avoiding

the unnecessary burning of power in the front end transducer.

When taken back out of power-down, and the power switch is

closed, the user should ensure that the front end circuitry is fully

settled before attempting a read from the AD7709.

The circuit in Figure 19 shows a method that utilizes three

pseudo-differential input channels on the AD7709 to temperature-

compensate a pressure transducer.

OUT(+)

OUT(–)

IN(–)

IN(+)

PRESSURE

BRIDGE

XTAL1

XTAL2

IOUT1

6.25k

VDD

REFIN(+)

REFIN(–)

AIN2

AIN1

AIN3

AINCOM

GND

AD7709

250

Figure 19. Temperature-Compensating a Pressure

Transducer

In this application, pseudo-differential input channel AIN1/

AINCOM is used to measure the bridge output while pseuo-

differential channels AIN2/AINCOM and AIN3/AINCOM

measure the voltage across the bridge. The voltage measured

across the bridge will vary proportionally with temperature,

and the delta in this voltage can be used to temperature-

compensate the output of the pressure bridge.

Temperature Measurement

The AD7709 is also useful in temperature measurement appli-

cations. Figure 20 shows an RTD temperature measurement

application.

REFIN(–)

IOUT1

6.25k

AIN2

AIN1

AD7709

REFIN(+)

IOUT2

VDD

GND

PWRGND

DRDY

SCLK

DIN

DOUT

XTAL1

XTAL2

RREF

RL1

RL2

RL3

RL4

RCM

RTD

200 A

CONTROLLER

Figure 20. 4-Wire RTD Temperature Measurement

Using the AD7709

In this application, the transducer is an RTD (Resistive Tem-

perature Device), a PT100. The arrangement is a 4-lead RTD

configuration. There are voltage drops across the lead resistances

RL1 and RL4, but these simply shift the common-mode voltage.

There is no voltage drop across lead resistances RL2 and RL3

since the input current to the AD7709 is very low, looking into a

high input impedance buffer. RCM is included to shift the analog

input voltage to ensure that it lies within the common-mode

range (GND + 100 mV to VDD – 100 mV) of the ADC. In the

application shown, the on-chip 200

mA current source provides

the excitation current for the PT100 and also generates the reference

voltage for the AD7709 via the 6.25 k

W resistor. Variations in

the excitation current do not affect the circuit since both the

input voltage and the reference voltage vary ratiometrically with the

excitation current. However, the 6.25 k

W resistor must have a low

temperature coefficient to avoid errors in the reference voltage

over temperature.

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