I have problems to get real values at lowest voltages on ADC pin because I am using an OPAMP for symetric source (UA741) and I'm using ground for the negative DC PIN.

I can resolve this by two methods:

1.- Waiting for LM358 I buy recently which handle single supplies.

2.- Adding voltage biasing and correct received values though software but I loss usable resolution.

Falstad simulation

Geogebra graphic function