Serial digital data communications receiver with improved automatic cable equalizer, AGC system, and DC restorer

A serial digital data communications receiver with an improved automatic cable equalizer that is less susceptible to jitter and has greater multi-standards capability, and an improved automatic gain control system with a DC restorer that provides optimal edge jitter performance while avoiding the possibility of a latch-up condition at the start of data transmission. The automatic cable equalizer for equalizing signals received over cables of different lengths has multiple stages each having a transfer function of 1+Ki[fi(j)] wherein each of the Ki vary in accordance with a sequential gain control methodology. The AGC system uses the difference between band-pass filtered versions of the amplitudes of the input and output of a DC restorer based on quantized feedback, to regulate the AGC circuit. The DC restorer comprises a comparator for generating a quantized output and further clamps the input with a clamping circuit so that a version of the quantized output is fed back to the input while avoiding the possibility of operational failure of the comparator at the onset of data transmission.