A CMOS photodiode array with in-pixel data acquisition system for computed tomography

A CMOS photodetector array with in-pixel electronics has been developed for computed tomography (CT) applications. Current CT detectors are based on two discrete components: a photodiode array and a data acquisition system (amplifier). Both elements have to fullfill a series of severe requirements. CT scanners are moving toward larger detectors and higher speed, and yet lowering costs and improving performance. This contribution relates to the integration of both elements into a standard CMOS process to fullfill future CT scanner specifications and for a cost-effective solution. A series of limitations have been overcome to integrate both the photodiode and a charge-sensing amplifier at pixel level. In order to balance the limited responsitivity of standard CMOS photodiodes, a new low-capacitance device has been devised so that low-noise design is possible while providing enough gain in the amplifier. Since a good geometric detective quantum efficiency is desired (>60%), the available area for electronics is very limited. In order to achieve a necessary dynamic range of 17 bits in such reduced area, a single-stage amplifier with automatic gain-switching has been designed to achieve a quantum limited system under CT operating conditions.