LTC1658 View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1658

14-Bit Rail-to-Rail Micropower DAC in MSOP

Linear Technology 

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TYPICAL APPLICATIO S

An Optoisolated 4mA to 20mA Process Controller

LT®1121-3.3

IN

OUT

+

1µF

237k

1% 20k

FROM

OPTO-

ISOLATED

INPUTS

VCC

REF

CLK

60.4k

1% 5k

DIN LTC1658 VOUT

CS/LD

3.01k

1%

+

LT1077

–

LTC1658

VLOOP

3.8V TO 30V

1k

Q1

2N3440

CLK

DIN

CS/LD

OPTOISOLATORS

3.3V

3.6k

500Ω

4N28

CLK

DIN

CS/LD

1658 TA04

RS

10Ω

IOUT

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

LT1121-3.3 is used for the 4mA offset current and VOUT is

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

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.

9

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