A CMOS imager with PFM/PWM based analog-to-digital converter

An On-pixel Analog-to-Digital Converter based on both PFM and PWM schemes is reported. The proposed architecture uses a limited number of transistors that can be implemented in a small silicon area resulting in a 23% fill-factor. The digital sensor can be extemally configured in order to operate under either the PFM or PWM scheme. At high-light intensities, the PFM scheme is replaced by the PWM scheme which proves to be much more efficient in terms of power consumption and clock frequency requirements. An in-built light adaptation mechanism has also been implemented which allows the sensor to better adapt to low-light intensity or to adjust the sensor saturation level. As a consequence, the sensor features a programmable dynamic range. Image lag is reduced in both schemes since a reset of the photodetector is performed after the conversion period. The pixel based ADC has been designed and fabricated using CMOS 0.25pm technology.