A 55µm×55µm charge-integration digital pixel sensor for digital direct mammography in 0.18µm CMOS technology

This paper presents a new compact and low-power CMOS DPS circuit for direct digital X-ray imagers. Based on charge-integration A/D conversion to avoid signal looses due to pile-up and charge-sharing effects, this novel DPS cell includes the following functionality: selectable electrons/holes collection, self-biasing and test capabilities, lossless charge-integration A/D conversion, individual gain tuning for about 1 decade of fixed-pattern-noise (FPN) cancellation, up to 20nA of dark current self-calibration cancellation, and a 100Mbps digital only I/O serial interface. Three generations of the proposed DPS have been integrated in 0.18μm 1P6M CMOS technology with 100μm, 70μm and 55μm pitch values, resulting in typical transfer gain factors of 1LSB/50ke^-, sampling rates from 1ms to 1s, and equivalent noise charge levels below 2ke_rms^-. Thanks to transistor sub-threshold operation and exhaustive circuit reuse, the full active pixel consumes only 6μW_rms at 1.8V supply voltage.