The use of AO in Extremely Large Telescopes, used to improve performances in smaller telescopes, becomes now mandatory to achieve diffraction limited images according to the large apertures. On the other hand, the new dimensions push the specifications of the AO systems to new frontiers where the order of magnitude in terms of computation power, time response and the required numbers of actuators impose new challenges to the technology. In some aspects implementation methods used in the past result no longer applicable. This paper examines the real dimension of the problem imposed by ELTs and shows the results obtained in the laboratory for a real modal wavefront recovery algorithm (Hudgin) implemented in FPGAs. Some approximations are studied and the performances in terms of configuration parameters are compared. Also a preferred configuration will be justified.