Design and Evaluation of a CMOS-Photosensor with In-Pixel Sigma-Delta Modulator for X-ray Computed Tomography

A multichannel SigmaDelta-type analog-to-digital converter has been designed for data readout in CT scanners. A test-structure with a 4 times 4 photodetector array has been fabricated in a 0.8 mum CMOS process. Aiming at an integrated solution towards a 'digital pixel', each sensor element is formed by a photodiode and a third-order SigmaDelta-modulator. The switched-capacitor circuitry has been realized as a fully differential-path design. Test measurements have been carried out and analyzed in terms of dynamic range and noise. Simulations on layout level have been performed. A digital post-processing scheme has been devised for decimation and low-pass filtering of the modulator outputs