Abstract
An optimized digital shaping filter has been developed for the GERDA experiment which searches for neutrinoless double beta decay in $^{76}$Ge. The GERDA Phase I energy calibration data have been reprocessed and an average improvement of 0.3 keV in energy resolution (FWHM) at the $^{76}$Ge Q value for $0\nu\beta\beta$ decay is obtained. This is possible thanks to the enhanced low-frequency noise rejection of this Zero Area Cusp (ZAC) signal shaping fillter.