AD7719 datasheet(36/40 Pages) AD

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REV. 0

AD7719

–36–

Pressure Measurement

One typical application of the AD7719 is pressure measure-

ment. Figure 19 shows the AD7719 used with a pressure

transducer, the BP01 from Sensym.

The pressure transducer is arranged in a bridge network and

gives a differential output voltage between its OUT(+) and

OUT(–) terminals. With rated full-scale pressure (in this case

300 mmHg) on the transducer, the differential output voltage is

3 mV/V of the input voltage (i.e., the voltage between its IN(+)

and IN(–) terminals).

Assuming a 5 V excitation voltage, the full-scale output range

from the transducer is 15 mV. The excitation voltage for the

bridge can be used to directly provide the reference for the ADC

as the reference input range includes the supplies. Alternatively,

a suitable resistor divider can be implemented that allows the

full dynamic range of the input to be utilized in these applica-

tion. This implementation is fully ratiometric, so variations in

the excitation voltage do not introduce errors in the system.

Choosing resistor values of 20 k

Ω and 12 kΩ as per Figure 19

give a 1.875 V reference voltage for the AD7719 when the exci-

tation voltage is 5 V.

AD7719

IN+

OUT+

OUT–

IN–

20k

12k

EXCITATION VOLTAGE = 5V

AVDD

DVDD

AIN1

AIN2

REFIN1(+)

PWRGND

DGND

AGND

REFIN2(–)

Figure 19. Pressure Measurement Using AD7719

Using the part with a programmed gain of 128 results in the

full-scale input span of the AD7719 being 15 mV, which corre-

sponds with the output span from the transducer.

A second key advantage to using the AD7719 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 of con-

cern, the AD7719 can be put in 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 cir-

cuitry is fully settled before attempting a read from the AD7719.

The circuit in Figure 20 shows a method that utilizes all three

pseudo-differential input channels on the AD7719 main channel

to temperature-compensate a pressure transducer.

OUT(+)

OUT(–)

IN(–)

IN(+)

PRESSURE

BRIDGE

XTAL1

XTAL2

IOUT1

6.25k

AVDD

REFIN(+)

REFIN(–)

AIN2

AIN1

AIN3

AIN4

AGND

AD7719

250

Figure 20. Temperature-Compensating a Pressure

Transducer

In this application, pseudo-differential input channel AIN1/AIN4

is used to measure the bridge output while pseudo-differential

channels AIN2/AIN4 and AIN3/AIN4 measure the voltage

across the bridge. The voltage measured across the bridge will

vary proportionally with temperature, and the delta in this volt-

age can be used to temperature-compensate the output of the

pressure bridge.

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