Hi Nishant, thanks to have checked this out. Finally I found the right combination of C, Rs and frequency excitation required to get a resonable signal out. I am not strong with the theory of Op-Amps but I read some books and I udnerstood where the problem was.
BTW, for other users that may find this useful, my actual parameetrs are f=5kHz, R1=1000kOhm, R2=10kOhm, C=1 uF. The sin wave is sampled at 1MhZ