AD7705 datasheet(40/44 Pages) AD

40 / 44 page

AD7705/AD7706

Rev. C | Page 40 of 44

SMART TRANSMITTERS

Another application where the low power, single-supply, 3-wire

interface capabilities of the AD7705/AD7706 are beneficial is in

smart transmitters. Figure 28 shows a block diagram of a smart

transmitter using the AD7705. Because a smart transmitter

must operate from a 4 to 20 mA loop, tolerances in the loop

mean that the current available to power the transmitter can be

as low as 3.5 mA. The AD7705 consumes only 320 μA, leaving

at least 3 mA available for the rest of the transmitter. The

AD7705, with its dual-input channel, is ideally suited for

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

μF

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

μF

0.01

μF

0.1

μF

2.2

μF

DN25D

1000pF

0.01

μF

0.01

μF

0.0033

μF

ISOLATION

BARRIER

SENSORS

RTD

MAIN TRANSMITTER ASSEMBLY

4mA

20mA

Figure 28. Smart Transmitter Using the AD7705

AD7705 Datasheet(PDF) 40 Page - Analog Devices

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