The performances of A/D converters are usually characterized in their granular region, by adopting amplitude limited sine-wave testing signals. However, some applications, like digital telecommunication systems, often require the conversion of signals which noticeably differ from sine-waves and may introduce ADC overloading phenomena. In this paper, the A/D conversion of Gaussian distributed signal is investigated, both for PCM and sigma-delta converters. Then, the effects of quantization noise upon the behavior of a digital communication system are considered. By extending previous results; a model is introduced, which describes the BER performance of an OFDM system also in presence of overloading quantization noise, both for PCM and sigma-delta A/D converters.