A new IC with level-crossing ADC readout architecture for PET detector signals

A technique often used in Positron Emission Tomography (PET) systems to readout pulse signals from detectors is to use a free-running ADC to digitize the pulse waveform and then use software to determine the time and energy of the pulse. The advantage of this technique over analog-based methods is the possibility of obtaining superior time resolution with software-based time discrimination algorithms and the flexibility of working with digitized values. Unfortunately, the digitized pulse waveform output from a free-running ADC is an inefficient representation of the signal. As illustrated in the paper, the rising edge, where we desire fine sampling for arrival time determination, ends up being sampled coarsely. The falling edge, which is only used for energy determination, can be coarsely sampled, but we fine sample it instead, so this solution is not cost or power optimal. The rest of this paper describes a chip which samples at user-defined voltage levels to overcome these issues.