AD7709 datasheet(29/33 Pages) AD

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REV. A

AD7709

–28–

MICROCONTROLLER

VCC

GND

REF OUT2

REF IN

10 F

0.1 F

VDD

AIN1

AIN2

DOUT

SCLK

DIN

GND

REFIN(+)

REFIN(–)

0.1 F

REF OUT1

CLOCK

LATCH

DATA

4.7 F

COM

AD7709

AD421

BOOST VCC

0.01 F

1000pF

LOOP

POWER

10 F

3.3V

1.25V

DN25D

0.01 F

LOOP

RTN

COMP

DRIVE

VARIABLES

AIN3

AIN4

Figure 23. Smart Transmitter Employing the AD7709

Smart Transmitters

Smart transmitters are another key design-in area for the AD7709.

The

converter, single-supply operation, 3-wire interface

capabilities, and small package size are all of benefit in smart

transmitters. Here, the entire smart transmitter must operate

from the 4–20 mA loop. Tolerances in the loop mean that the

amount of current available to power the transmitter is as low as

3.5 mA. Figure 23 shows a block diagram of a smart transmitter

that includes the AD7709.

Not shown in Figure 23 is the isolated power source required to

power the front end.

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