I'm trying to desing a 4-20mA receiver for arduino and I have a couple of questions. first, this is what I base my desing on:

First, I think I understand most of the design, except for the R5 resistor... I'm not sure but is this resistor divide by 2 the input on the U1B ?

Secondly, that design work with AVR board, but I want to make it compatible with 3.3v base board. I can't have a lower than 1 gain so another solution is to put a serial resistor like R5 to divide the voltage at the input of U1B ( that's were this resistor bugging me).

another thing, if not 4-20 device is connected to the input receiver, the output , with the 1V to 0 offset, will be -1V. so to protect the board, can I put a reverse diode from ground to the receiver output or at the output of the U1B ? or a better solution...

Thanks

Ok just done my math… and understant the R5 function.

That’s leave me only with the potection diode… What can I do… ?

nitrof: Ok just done my math... and understant the R5 function. That's leave me only with the potection diode... What can I do.. ?

Regarding "Figure 3"?

nitrof:
…that design work with AVR board, but I want to make it compatible with 3.3v base board.

Use a 51ohm 1% metalfilm resistor for the current to voltage conversion.
And a 10k resistor between the 51ohm resistor and analogue-in for pin protecton.
A 100n cap from analogue pin to ground might help in noisy environments.
Post a diagram if you’re not sure.

Read the 0.204 to 1.020volt across the 51ohm resistor with Arduino’s more stable ~1.1volt Aref.
https://www.arduino.cc/en/Reference/AnalogReference
Works on 3.3volt and 5volt Arduinos.
Leo…

That's what I have done so far...