Thank you wvmarle and Wawa for you replies.
I spent sometime today and study little bit more about how ADC works in Arduino and how to leverage resistance ratio.
With a circuit as described above, I can derive the Rt reading the ADC value (0..1023), knowing R1:
Rt=R1/ADC(1024-ADC)
Then I have calucalted A, B and C coefficent of Steinhart-Hart formula, based on three known values of Rt at 3 Temps (-10°C, 10°C, 30°C).
This allows me to derive the Temperature given Rt with an accuarcy of less than +/-0,02°C in that range, which is fine.
The PTC is going to be used in a house on the mountains to check temp remotly and switch heating on.
The expected range of temp is -5°C..+25°C (delta=30°C).
According to this value, and following Wawa, I will get a 33,3 ADC difference.
That is 0,9°C per ADC step.
Is there any way to get a better accuracy, let's say 0,5°C per ADC step?
Thanks for your support
Andrea