AD7715 datasheet(39/41 Pages) AD

39 / 41 page

AD7715

Data Sheet

Rev. E | Page 38 of 40

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 AD7715 consumes only 450 μA, leaving 3 mA available for

the rest of the transmitter. Figure 17 shows a block diagram of a

smart transmitter which includes the AD7715. Not shown in

Figure 17 is the isolated power source required to power the

front end.

INPUT/OUTPUT

STAGE

SIGNAL

CONDITIONER

MICROCONTROLLER UNIT

*PID

*RANGE SETTING

*CALIBRATION

*LINEARIZATION

*OUTPUT CONTROL

*SERIAL COMMUNICATION

*HART PROTOCOL

VOLTAGE

REFERENCE

MCLK

OUT

AD7715

ISOLATED SUPPLY

ISOLATION

BARRIER

SENSORS

RTD

ohm

LOOP

RTN

MCLK

COM

4mA

20mA

DVDD AVDD

REF IN

AGND

DGND

ISOLATED GROUND

COM

MAIN TRANSMITTER ASSEMBLY

BANDPASS

FILTER

WAVEFORM

SHAPER

HART

MODEM

BELL 202

DAC

VOLTAGE

REFERENCE

VOLTAGE

REGULATOR

VCC

Figure 17. Smart Transmitter Using the AD7715

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