Digital vision sensor with pixel level analog-to-digital converter

Abstract

In this paper we propose an on-pixel Analog-to-Digital converter based on pulse frequency modulation (PFM) scheme. This PFM based converter presents a very viable solution for pixel level based ADC. It uses a very simple and robust circuit that can be implemented in a compact area resulting in a 23% fill-factor. The low-light performance of the PFM converter is improved by using a wider counting period. By modifying the counting period, it is possible to change the saturation level of the ADC and hence improve the dynamic range of the sensor. Image lag is eliminated in the PFM On-pixel ADC since a reset of the photodetector is performed after the conversion period. In addition, the PFM on-pixel ADC has a very important advantage: it is insensitive to variations of the supply and reference voltages. The pixel based ADC has been designed and fabricated using CMOS 0.25 μm technology.