Considering melanie's and dhouston, last suggestions, I think the simplest, cheapest, noise cleanest, circuit simplest and MIPS faster option is to just connect sensors directly to my MCU/PIC/CPU.. specifically considering this case in which my PIC has a 12 bit resolution A/D (4 times the resolution than 10 bit). I'm happy with the idea, will test it and I'll let you know how how it works! and I think I'll can live with it

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PD: Just away from this, appart I will play with OpAmps, just to know how these toys work.

RodSTAR