AD7719 datasheet(39/40 Pages) AD

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AD7719

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Smart Transmitters

Smart transmitters are another key design-in area for the AD7719.

The dual sigma-delta converter, single supply operation, 3-wire

interface capabilities, and small package size are all of benefit is

in smart transmitters. Here, the entire smart transmitter must

operate from the 4 mA-to-20 mA loop. Tolerances in the loop

mean that the amount of current available to power the trans-

mitter is as low as 3.5 mA. Figure 24 shows a block diagram of a

smart transmitter which includes the AD7719.

MICROCONTROLLER

REF OUT2

REF IN

10 F

0.1 F

AVDD

DVDD

AIN1

AIN2

DOUT

SCLK

DIN

DGND

AGND

AIN5

AIN5 COM

REFIN2

REFIN(+)

REFIN(–)

0.1 F

REF OUT1

CLOCK

LATCH

DATA

4.7 F

COM

VCC

GND

AD7719

AD421

BOOST VCC

0.01 F

1000pF

LOOP

POWER

10 F

3.3V

1.25V

DN25D

0.01 F

LOOP

RTN

COMP

DRIVE

MAIN

VARIABLES

SECONDARY

VARIABLES

AIN3

AIN4

Figure 24. Smart Transmitter Employing the AD7719

Not shown in Figure 24 is the isolated power source required to

power the front end. The advantages of the AD7719 in these

applications is the dual channel operation, meaning that the

user does not have to interrupt the main channel when measur-

ing secondary variables, and therefore does not have the latency

associated with the settling times of the digital filter. The fact

that the AD7719 is factory-calibrated means that in the majority

of applications, the user will not have to perform any field cali-

bration given the excellent offset and gain drift performance of

the device as a result of the signal chain chopping employed in

the signal chain.

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