[hvalentim]

This 9V (more or less) supply voltage doesn't have any effect on the ADC reference voltages used by the Arduino. This only depends on the quality of it's own 5V regulator, which you can't control (at least keeping the Arduino board untouched).

Yes. But regardless of individual units, by design, the Arduino voltage regulator itself is a linear regulator which should perform in a similar fashion/have the same caveats as the L7809. We actually have two linear regulators applied in sequence.

It seems that you are powering it with a problematic power source.

That particularity is the core of the matter.

Try adding a large electrolytic capacitor (1000uF or more) to input an then check the results.

I have one single 100uF available. Can try it tomorrow.

you can spend the almost 20€ that a TR1- 2490 will cost

Actually it seems 7,6 euros + VAT will suffice.

Anyway, the same result can be obtained, spending less than 1€, with an external DC/DC converter module. Wiring it instead of a 7809 is very easy.

Thanks for the tip.

Most of the times, the better is keep the things simple!

You will see in the end I will go for the "simple" solution. I just don't take it without questioning/understanding exactly why