AD7714YN datasheet(34/40 Pages) AD

34 / 40 page

AD7714

REV. C

–34–

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/Volt 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 is also used to generate the reference voltage for the

AD7714. Therefore, variations in the excitation voltage do not

introduce errors in the system. Choosing resistor values of

24 k

Ω and 15 kΩ as per the diagram give a 1.92 V reference

voltage for the AD7714 when the excitation voltage is 5 V.

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

scale input span of the AD7714 being 15 mV which corresponds

with the output span from the transducer.

APPLICATIONS

The on-chip PGA allows the AD7714 to handle an analog input

voltage range as low as 10 mV full-scale with VREF = +1.25 V.

The differential inputs of the part allow this analog input range

to have an absolute value anywhere between AGND and AVDD

when the part is operated in unbuffered mode. It allows the user

to connect the transducer directly to the input of the AD7714.

The programmable gain front end on the AD7714 allows the

part to handle unipolar analog input ranges from 0 mV to

+20 mV to 0 V to +2.5 V and bipolar inputs of

±20 mV to

±2.5 V. Because the part operates from a single supply these

bipolar ranges are with respect to a biased-up differential input.

Pressure Measurement

One typical application of the AD7714 is pressure measure-

ment. Figure 12 shows the AD7714 used with a pressure trans-

ducer, the BP01 from Sensym. The pressure transducer is

CLOCK

GENERATION

OUT+

IN+

OUT–

IN–

AUTO-ZEROED

MODULATOR

CHARGE BALANCING A/D

CONVERTER

DIGITAL

FILTER

AVDD

1 A

BUFFER

AVDD

SERIAL INTERFACE

STANDBY

SYNC

MCLK IN

MCLK OUT

1 A

REF IN (+)

REF IN (–)

AGND

DGND

BUFFER

DOUT

DIN

SCLK

POL

DRDY

RESET

24k

15k

DVDD

AD7714

AGND

+5V

EXCITATION VOLTAGE = +5V

AIN1

AIN2

AIN3

AIN4

AIN5

AIN6

A = 1–128

PGA

Figure 12. Pressure Measurement Using the AD7714

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