4-20mA current loop transmitter

Thread Starter


Joined Sep 24, 2013
I am trying to design a 4-20mA current loop for a project. I have a circuit that is nearly working however I cannot get the current below 5.5mA. I attached my circuit. I used an analog devices circuit as a reference which I also attached. The portion of the analog devices circuit I am using starts at R9.

At 5V input I get a current of 20mA, but at 0V input I get a current of 5.61mA. I have tried changing all of the resistors, and I have tried a couple different op amps with no luck.

Any help would be appreciated.




Joined Sep 9, 2010
What are you using for V1?

Are you getting more-or-less linear current-vs-voltage but you can't get down to 4mA?


Joined Jun 6, 2011
The LM340 has a quiescent current greater than 5 ma, even at a low 10 mA load. Also, the LM358 is probably running around 2 ma. The total current consumption of the transmitter cannot exceed 4 ma at the max loop voltage, or the transmitter will not work properly.