Dewalt:
I have connect 2 op-amp togeter (LM324N) and I don´t get the right result. What do u recommend? what ap-amp shuld I use and how to connect it?
post your circuit diagram.
off hand, i'd say even 1 op-amp is more than adequate... you need to however do some work to design the bias and gain within swing range... say bias your signal to 1.7V as the midpoint.... and put a gain of 60 odd... and feed 3.5v to the analog ref in arduino to get max range.
working with 2 opamps has its benefits but needs far more careful design work