I think you misunderstand what I was getting at.
I don't want to to accept a 4-20ma input from a 4-20ms sensor into the arduino.
I want to use the arduino to simulate a 4-20ma sensor. e.g.
- Connect a microphone to the arduino and output a 4-20 output according to the volume.
or - Connect a colour sensor to the arduino and output a 4-20 output according to the proportion of red light.