  Digital pixel sensor (DPS) has evolved as a pivotal component in modern
imaging systems and has the potential to revolutionize various fields such as
medical imaging, astronomy, surveillance, IoT devices, etc. Compared to analog
pixel sensors, the DPS offers high speed and good image quality. However, the
introduced intrinsic complexity within each pixel, primarily attributed to the
accommodation of the ADC circuit, engenders a substantial increase in the pixel
pitch. Unfortunately, such a pronounced escalation in pixel pitch drastically
undermines the feasibility of achieving high-density integration, which is an
obstacle that significantly narrows down the field of potential applications.
Nonetheless, designing compact conversion circuits along with strategic
integration of 3D architectural paradigms can be a potential remedy to the
prevailing situation. This review article presents a comprehensive overview of
the vast area of DPS technology. The operating principles, advantages, and
challenges of different types of DPS circuits have been analyzed. We categorize
the schemes into several categories based on ADC operation. A comparative study
based on different performance metrics has also been showcased for a
well-rounded understanding.
