AD7798 datasheet(26/28 Pages) AD

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AD7798/AD7799

Data Sheet

Rev. B | Page 26 of 28

APPLICATIONS INFORMATION

The AD7798/AD7799 provide a low cost, high resolution

analog-to-digital function. Because the analog-to-digital

function is provided by a ∑-∆ architecture, the parts are more

immune to noisy environments, making them ideal for use in

sensor measurement and industrial and process-control

applications.

WEIGH SCALES

Figure 19 shows the AD7798/AD7799 being used in a weigh

scale application. The load cell is arranged in a bridge network

and gives a differential output voltage between its OUT+ and

OUT– terminals. Assuming a 5 V excitation voltage, the full-

scale output range from the transducer is 10 mV when the

sensitivity is 2 mV/V. The excitation voltage for the bridge can

be used to directly provide the reference for the ADC because

the reference input range includes the supply voltage.

A second advantage of using the AD7798/AD7799 in transducer-

based applications is that the 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.

In applications where power is of concern, the AD7798/AD7799

can be placed in standby mode, thus significantly reducing the

power consumed in the application. In addition, the low-side

power switch can be opened while in standby mode, thus

avoiding unnecessary power consumption by the front-end

transducer. When the part is taken out of standby mode and the

low-side power switch is closed, the user should ensure that the

front end circuitry is fully settled before attempting a read from

the AD7798/AD7799.

In Figure 19, temperature compensation is performed using a

thermistor. In addition, the reference voltage for the temperature

measurement is derived from a precision resistor in series with

the thermistor. This allows a ratiometric measurement—that is,

the ratio of the precision reference resistance to the thermistor

resistance is measured; therefore, variations of the reference

voltage do not affect the measurement.

DOUT/RDY

DIN

SCLK

DVDD

SERIAL

INTERFACE

AND

CONTROL

LOGIC

Σ-Δ

ADC

AD7798/AD7799

AIN2(+)

AIN2(–)

AIN1(+)

AIN1(–)

AVDD

GND

MUX

INTERNAL

CLOCK

GND

AVDD

REFIN(–)

REFIN(+)

PSW

IN-AMP

REFERENCE

DETECT

IN+

IN–

OUT–

OUT+

AVDD

Figure 19. Weigh Scales Using the AD7798/AD7799

AD7798 Datasheet(PDF) 26 Page - Analog Devices

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