9

LTC1658

An Optoisolated 4mA to 20mA Process Controller

CLK

CS/LD

1

µF

IN

OUT

Q1

2N3440

10

Ω

3.8V TO 30V

REF

FROM

OPTO-

ISOLATED

INPUTS

LTC1658

1658 TA04

3.01k

1%

–

+

+

LT1077

1k

20k

237k

1%

5k

60.4k

1%

OPTOISOLATORS

500

Ω

3.3V

3.6k

4N28

CLK

CS/LD

CLK

CS/LD

This circuit shows how to use an LTC1658 to make an

optoisolated digitally controlled 4mA to 20mA process

controller. The controller circuitry, including the

optoisolation, is powered by the loop voltage that can have

a wide range of 3.8V to 30V. The 3.3V output of the

is a sense resistor and the op amp modulates the transis-

tor Q1 to provide the 4mA to 20mA current through this

resistor. The potentiometers allow for offset and full-scale

adjustment. The control circuitry dissipates well under the

4mA budget at zero-scale.

TYPICAL APPLICATIO S