Thanks for the link Terrence.
I´ve found the problem, but it seems a little strange to me. I´ve made a mistake and called a ISR from the delta_sigmaINT.asm.
The ISR is of another module, and when I enabled the AD, everything stopped working. I suppose the interrupt calling was breaking my program. I hadn´t enable the AD interrupt, so I found this behavior strange. Is it coherent?