Hi Marco,
Thanks for the schematic. I think your resistors are responsible for the noise that you are seeing. Resistors contribute 4nV/rtHz times the square root of the resistor value in kilo ohms ( sqrt(R in kohms)*4nV/rtHz). When referred to output, these resistor noise will be multiplied by the gain of each gain block of your system, depending on its location. Based on your resistor values, I calculated at least 3.5mVp-p noise after the RC filter, which I think is close to the +/-2mV that you are seeing. There are 2 things you can consider to lower this noise. First is to lower your resistor values and second, lower the cutoff frequency of the RC filter at the ADC input.
There are video tutorials by ADI available on YouTube that discusses noise on amplifiers. Here's the link.
I hope this helps!
Regards,
Neil
