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.

computer controlled current loop circuit schematic

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]