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In DVB-S2 receiver, the demodulator needs accurate soft information about the received symbol distance from the reference constellation points. Digital automatic gain control is adopted to adjust the level of the incoming symbols to the reference constellations to help obtaining a good demodulator performance. In this paper, an optimized digital automatic gain control method for DVB-S2 system is presented based on the analysis of a novel reasonable cost function and the potential constellation property. The proposed scheme can operate adequately well over the demanded range of signal-to-noise ratio and support both with pilot and without pilot modes with much lower complexity. The mathematical derivation of the scheme is given in detail and the advantages over the conventional methods are clearly demonstrated by simulation results.