A CMOS Image Sensor using Variable Reference Time Domain Encoding

In this paper, a variable reference time domain encoding CMOS image sensor is presented. The time domain encoding vision sensor is known to suffer from slow conversion time, especially at low level of illumination. This is due to limited photocurrent generated to discharge the photodiode junction voltage to the reference voltage. A variable referencing scheme is proposed so that the reference voltage will be modulated and bounded by a specified deadline. The pixel consists of a photodiode, an analogue comparator, an 8-bit SRAM, a SR latch, and occupies an area of 32mum times 35mum, with a fill-factor of 12.6 % using a 0.35mum CMOS process. Simulation results show that signal conversion can be achieved by using pre-defined threshold voltages. By using four levels of reference voltage and 1/10 of the original conversion time required by the original time domain encoding, over 70% reduction in total integration time can be achieved.