Isolated current loop receiver design

isolated current loop receiver

probably you already know siemens application note regaring IL300 linear opto-isolator, I implemented same circuit (same amplifiers, TIL300 optocoupler , IL300 from another manufacturer, same FET) but my circuit is not linear as described in the app. note.

I suppose the only difference is they fed opamps with +/-9V DC, but I applied 12V DC.

I measure input current with a Fluke multimeter and I am using an oscilloscope to measure DC output of isolated part.
For input, I used a simple potentiometer and voltage source.

Here are some measurements :
input current - output voltage
20 mA - 11.4V
5 mA - 4.52V

As you can see it is not linear. I am not good in analog, actually I am firmware guy

here is the app. note:
hxxp://crio.mib.infn.it/wig/electronics/Componenti/Siemens%20e%20Infinenon/Accoppiatori%20Ottici/IL300%20per%20industria.pdf

There is a pot. for calibration:
"" The receiver was calibrated by adjusting
R6 for 3.00 V output with a loop current of 12.00 mA at
TA=23°C""

I suppose it is only about DC offset and should not affect linearty? Am I right?

Regards

Plot several points across the full scale and see if they make a straight line. If they do, you can always add a final op amp with a gain and offset to make the scale factor whatever you want.

I already measured some points, input current/output voltage ratio is not fixed...

So I If they are not linear what can I do to make it fixed?
There is a LED (not coupler's LED) for visual indication and voltage drop at the input, does it affect linearity?

Is it important to use +/-9V or +/-12V?

c.u.

Original app. note needs 100K resistor and 50K trimpot in series for gain of transimpedance amplifier at the output. They recommend to adjust output voltage 3V for 12mA loop current...

But I could adjust it with 22K resistor and 20K trimpot in series... 100K+50K is out of this range...

What is wrong?