Biasing an AC signal for sampling - detect minor changes via Arduino
would a 0.1v change be detected at the ADC ?


If the analogue signal range of your ADC is from 0 volts to +5 volts then biasing the AC signal to 2.5 volts is a good start. If your AC signal can be quite large and possibly cause a peak outside the range 0 volts to +5 volts then you need to attenuate. You have some attenuation; R1 and R2 and R3 form a 3:1 attenuator. If you need more then you need to make R1 bigger.

If the ADC has a digital resolution of 10 bits then \$2^{10}\$ = 1024

So your analogue resolution is 5 volts / 1024 = 4.883 mV.

This is the smallest change in analogue signal you can detect.