Computer-Controlled 4-20mA Current Loop

In some industries, to control the some equipments they are use computer-controlled 4-20mA current loop. This current loop is used transmit  a signal over a long distance wires. With current loop, the signal will be resistant from noise interference.

To operate this  computer-controlled 4 – 20mA we need a single power supply with a range of  3.3 to 30V. On the diagram there are R1,R2,R3,R4. R1 is used to set the zero reference of 4 mA and to calibrate we are use R2. R3 is used to set the full scale current output (20mA) and to calibrate it we use R4.
To calibrate the circuit there are two step,they are:

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

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