AD421

–12–

REV. C

REF IN

CS

DATA OUT

SCLK

DATA IN

AGND

DGND

MCLK IN

MCLK OUT

AD7714/

AD7715

ANALOG

TO

DIGITAL

CONVERTER

SENSORS

RTD

mV

TC

REF OUT1

BOOST

LV

COMP

DRIVE

LOOP

RTN

REF OUT2

REF IN

CLOCK

LATCH

DATA

COM

C1

C2

C3

LOOP

POWER

0.01 F

DN25D

2.2 F

3.3V

1.25V

4.7 F

AMBIENT

TEMP

SENSOR

AD421

MICROCONTROLLER

GND

WAVEFORM

SHAPER

BANDPASS

FILTER

HART

MODEM

BELL 202

0.1 F

0.01 F

1k

1000pF

HT20C12/20C15

4.7 F

100k

Figure 17. AD421 in HART Transmitter Application

across R2. The ratio of R1 to R2 determines the current that

current programmed to the AD421. R1 and R2 are external to

the AD421 and will need to be matched resistors to obtain a

highly accurate current source.

Current Source

Figure 18 shows an application circuit for the AD421 being

used as a current source. The current programmed to the

AD421 (4 mA to 20 mA) will develop a voltage across R1.

This same voltage due to negative feedback will be generated

LOOP

RTN

R2

R1

C1

C2

C3

COM

LV

2.2 F

COM

DRIVE

COMP

BOOST

0.01 F

0.01 F

0.0033 F

LATCH

CLOCK

DATA

REF IN

REF OUT2

REF OUT1

4.7 F

COM

AD421

+5V

10k

10k

10k

CLOCK

LATCH

DATA

5 VOLT

REGULATOR

OUT

IN

COM

Figure 18. AD421 in Programmable Current Source/Sink