Combined 1- and 2-bit sigma-delta-modulation-based read-out system for gamma-ray detectors

Photosensor waveform signals from gamma-ray detectors are digitized when estimation tasks that require pulse-shape information are going to be performed (such as pile-up detection, pulse-shape discrimination, and maximum-likelihood energy and timing estimation). Conventional analog-to-digital converters (ADC) require sampling frequencies on the order of hundreds of MS/s for scintillation decay times on the order of tens of ns. Taking into account typical ADC resolutions of 12 to 16 bits, the amount of data that needs to be stored, scaled by the large number of channels in SPECT and PET gamma-ray detectors, results in very complex read-out electronics. We present a novel multichannel read-out system that largely reduces electronics complexity, based on the combination of 1-bit and 2-bit sigma-delta modulation ADC, from which we are able to obtain energy, timing and pulse-shape information. We have proposed a novel technique based on 2-bit sigma-delta modulation in which the four possible output values (00, 01, 10 and 11) are not necessarily equally spaced. This means that the 2-bit DAC can be optimized for different pulse shapes, allowing us to extract as much information as possible from the minimum amount of data. We implement the read-out system on a modular gamma camera, which consists of a scintillator crystal and nine photomultiplier tubes (PMTs). Energy and position of interaction are estimated from the 1-bit modulation of individual PMTs. Simultaneously, the nine signals are summed and the 2-bit modulation is performed on the result. The analog part of the modulators is implemented on a printed circuit board with external analog comparators, and resistors that determine the feedback current. The digital side is implemented on a system on a chip (SoC) that incorporates an FPGA and a microprocessor in the same integrated circuit.