This work presents an architecture for CMOS active pixel sensors (APS) based on a novel lossless charge integration method, proposed for X-ray imagers in general but specifically optimized for full-field digital mammography. The objective is to provide all the required functionality inside the pixel, so to use full digital control and read-out signals only, therefore avoiding crosstalk between analog lines over large pixel arrays. It includes a novel lossless A/D conversion scheme besides a self-calibrating dark current cancellation circuit, a self-biasing circuitry, biphasic current sensing for the collection of electrons (e-) or holes (h+) and built-in test. Furthermore, FPN compensation is available by individually addressing the pixel's internal DAC controlling the gain. Implemented in a 0.18μm 1P6M CMOS technology with MiM capacitors, everything fits into a 70μm by 70μm due to the extensive reuse of available blocks and aggressive layout techniques. Also, thanks to the MOSFET subthreshold operation, the average power consumption is as low as 8μW/pixel.