9

LTC1451

LTC1452/LTC1453

sn145123 145123fas

An Isolated 4mA to 20mA Process Controller

Has 3.3V Minimum Loop Voltage

11451/2/3 TA04

3k

10k

1k

45k

5k

90k

5k

Q1

2N3440

10

Ω

3.3V TO 30V

OUT

IN

CLK

CS/LD

CLK

CS/LD

CLK

CS/LD

1

µF

LTC1453

4N28

OPTO-ISOLATORS

3.3V

500

Ω

LT

FROM

OPTO-

ISOLATED

INPUTS

+

–

LT1077

This circuit shows how to use an LTC1453 to make an

opto-isolated digitally controlled 4mA to 20mA process

controller. The controller circuitry, including the opto-

isolation, is powered by the loop voltage that can have a

wide range of 3.3V to 30V. The 1.22V 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.

Note that although these DACs have internal Schmitt

triggers and are suitable for use with slow rising edges

such as produced by the above optoisolator, the use of

optoisolators in a daisy-chained topology requires the

addition of a gate or the use of a fast isolator on the clock

guaranteed unless a clock edge with a rise time of less than

100ns is provided.

TYPICAL APPLICATIO S