AD7705 datasheet(30/32 Pages) AD

30 / 32 page

AD7705/AD7706

–30–

REV. A

AD7705

VDD

+5V

AIN1(+)

AIN1(–)

REF IN(–)

GND

MCLK IN

MCLK OUT

RESET

DRDY

DOUT DIN

SCLK

REF IN(+)

6.25k

400 A

RL1

RL2

RL3

RL4

RTD

Figure 24. RTD Measurement Using the AD7705

Smart Transmitters

Another area where the low power, single supply, three-wire

interface capabilities is 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 transmitter is as low as 3.5 mA.

The AD7705 consumes only 320

µA, leaving at least 3 mA

available for the rest of the transmitter. Figure 25 shows a block

diagram of a smart transmitter which includes the AD7705. The

AD7705 with its dual input channel is ideally suited to systems

requiring an auxiliary channel whose measured variable is used

to correct that of the primary channel.

VDD

GND

REF IN

ISOLATED SUPPLY

4.7 F

100k

MCLK IN

MCLK OUT

AD7705

ISOLATED GROUND

MICROCONTROLLER UNIT

•PID

•RANGE SETTING

•CALIBRATION

•LINEARIZATION

•OUTPUT CONTROL

•SERIAL COMMUNICATION

•HART PROTOCOL

VCC

COM

VCC

BOOST

REF OUT1

REF OUT2

REF IN

AD421

C1 C2 C3

COM

LOOP

RTN

COMP

DRIVE

4.7 F

0.01 F

0.1 F

2.2 F

DN25D

1000pF

0.01 F

0.0033 F

ISOLATION

BARRIER

SENSORS

RTD

MAIN TRANSMITTER ASSEMBLY

4mA

20mA

Figure 25. Smart Transmitter Using the AD7705

AD7705 Datasheet(PDF) 30 Page - Analog Devices

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