8

LTC1655

An Isolated 4mA to 20mA Process Controller

1655 TA03

3k

10k

1k

75k

1%

5k

150k

1%

20k

Q1

2N3440

10

Ω

6V TO 30V

OUT

IN

CLK

CS/LD

CLK

CS/LD

CLK

CS/LD

1

µF

LTC1655

4N28

OPTOISOLATORS

5V

500

Ω

LT

FROM

OPTOISOLATED

INPUTS

–

+

LT

This circuit shows how to use an LTC1655 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 6V to 30V. The 2.048V reference output of

is used for the digitally controlled 0mA to 16mA current.

transistor 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.