A digital CMOS imager with pixel level analog-to-digital converter and reconfigurable SRAM/counter

In this paper a CMOS image sensor with on-pixel analog-to-digital converter based on PWM scheme is proposed. The digital pixel sensor includes a novel digital circuit which allows to configure the internal 8-bit memory as a 4-bit counterlmemory, so as to reduce the data bit lines routed to each pixel from 8-bit to 4-bit. Hence the total parasitic capacitance as well as power consumption associated with the switching activities of the global data bus lines are reduced. Besides 8-bit precision, the imager can also be configured to 4-bit precision for low resolution applications, which has the advantage of increasing frame rate and decreasing overall power consumption. A prototype chip was realized in Alcatel 0.35mum CMOS technology. Each pixel occupies an area of 46mum x 48mum with a fill-factor of 14%.