4-20mA Current Loop Simulations

Thread Starter


Joined Oct 25, 2010
I am having trouble on a 4-20mA current loop circuit that I have prototyped (4 layer board). The circuit is not working properly. The output has large oscillations occurring at a regular intervals.

I am trying to determine the source of the issue. I do know that the opamp signal driving the transistor base is incorrect. It is a pulse wave instead of a static voltage. The bread board version I built works properly and shows a near static voltage.

I am looking for any suggestions on where to go with the issue. I thought I would try and simulate the circuit but am having trouble getting LTSpice to work with my negative voltage references. I've attached a copy of the pertinent portion of the actual circuit, and image of the simulation circuit that won't run and the LTSpice schematic and a snapshot of the transistor base voltage.

Thanks is advance.


best peter

Joined Oct 9, 2011
eng it, I have looked at your circuit and schematic and I am confused. Your circuit shows a short circuit between pins 2 & 3 of the ic. Is this correct?
What is the source of the circuit? (Where did you find this or is it your original design?) Linear Tech app note? If so, which one?
Is the pcb done or can you accommodate any changes? Did you come across this circuit in your research?
Happy to help but need more info. :)


Joined Jul 17, 2007
As best_peter mentions, I can't tell if you have everything shorted together at the input of the opamp, or if you meant for R23 to be connected to pin 3 and R17 to be connected to pin 2. Whichever way it is, you have R23 connected to ground, but that will cause the input to have a reference point that is below the IC's own ground; as Vcc_neg would be at some positive voltage due to the Vf of the forward-biased Zener diode U13.

Your LTSpice circuit is completely different from the other one. Are these supposed to be the same schematic or function?

I tried the LTSpice simulation. You didn't have a ground reference anywhere.
You're using the opamp with no feedback as an inverting amplifier. What you intended to do is anyone's guess.