Computer-Controlled 4-20mA Current Loop

Computer-controlled 4-20mA current loop is useful to control some industrial equipments.  By current loop, transmitting the signal over long distant should be resistant from noise interference.

This  computer-controlled 4 – 20mA is designed to operate on a single supply over a range of 3.3V to 30V.  The zero reference of 4 mA is set by R1 and calibrated using R2.  The full scale current output (20mA) is set by R3 and calibrated using R4. Here is the steps to calibrate the circuit:

• With a zero input code applied to the LTC1453 DAC,  set the output current IOUT to 4 mA  by adjusting R2
• With a full-scale code applied to the DAC, adjust R4 to get  the full-scale output current  20mA.

You can see only one opto-coupler section is drawn, actually you need three units of this block to interface the CLK, DIN, and CS/LD pins. [Circuit’s schematic diagram source: Linear Technology Application Notes]