A digitizer suitable for digitizing input signals having a high dynamic range. The digitizer is microprocessor controlled and comprises an input stage, a multi-channel attenuator or amplifier bank, a multiplexer and a analog-to-digital (A/D) converter. The function of the multi-channel bank is to move the input signal within the range of the A/D converter. The input signal to be digitized is fed through the input stage to each of the channels in the attenuator bank. The signal is scaled by each respective channel in the attenuator bank. The multiplexer is used to switch the scaled signal which is within the range of the A/D converter for digitizing. The digitized sample is then corrected according to the attenuation or gain factor of the scaled channel. The digitizer includes a comparator bank which is used to determine the channel with the widest signal range within the range of the A/D converter. The digitizer also includes a channel calibrator for calibrating the actual gain or attenuation of each channel. In another aspect, the digitizer can be implemented using an optical front-end comprising an optical beam splitter, optical attenuators and amplifier banks. The optical front-end provides improved noise immunity and faster speed of operation.