Design and test of an integrated Sigma–Delta analog-to-digital converter for X-ray Computed Tomography

Aiming at a compact, cost-effective data readout for X-ray CT, a monolithic approach is presented where photosensor and parts of the analog-to-digital converter are located on the same substrate. In a special realization of the ‘digital pixel’, a test array of photodiodes and in-pixel Sigma–Delta modulators has been developed and produced in a standard 0.8μm CMOS process. The single-bit, discrete-time modulator is of third order and has feedforward topology. Test samples have been characterized with emphasis on noise performance and linearity. An off-chip, decimation and low-pass filter has been designed according to the special requirements of CT. Signal processing with oversampling ADCs is compared to conventional solutions for data readout in X-ray CT.