Well,
I put together some stuff on a prototype pcb and servo routines worked ok! ( a little bit of improvement needed!)
I notice that without an op-amp and using just a max187 12bit ADC with EXTERNAL Vref tied to Vdd I get readings from ADC that plays between 2-3 value (NOT good). If we think that we use 12bit we have a resolution of about -+2meters and if the value plays between 2-3 values the error increase between -+6 meters (not good at all).
May be the only and better solution is go to 12bit ADC AND an op-amp too!
Also as a final thought is that maybe I have interference somewhere with the sensor although I used decoupling and filter capacitors according to MPX datasheet.
The main thing is that servo routines / speed controller arming e.t.c work fine!