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A conventional active pixel sensor (APS) uses a source follower (SF) in a pixel as a buffer. This SF is one of the major causes of nonlinearity, sensitivity degradation, and pixel readout noise. The proposed in-pixel comparing APS uses pixel transistors as a part of comparator for a single-slope ADC instead of using them as an SF. The prototype sensor was fabricated using a 0.35-mum 2P3M CMOS process. Experimental results show 15-times linearity improvement, 26% sensitivity enhancement, and 33% noise reduction over the conventional APS.