Event-based pulse frequency modulated (PFM) digital ROIC for infrared sensors

An infrared detector is limited by the charge-handling capacity of the integration capacitor in the readout integrated circuit (ROIC). This is especially true for longer light wavelengths, due to higher background photon flux and dark current. For that reason, pulse frequency modulated (PFM) digital ROICs (DROICs) have drawn attention from the infrared community. These encode light intensity into a pulse train by recycling the capacitor, with pixels continuously integrating their photogenerated charge and generating a pulse when the accumulated charge reaches a threshold. Conventional PFM DROICs embed a digital counter inside each pixel to accumulate the number of pulses. However, these in-pixel circuits impose area penalties that limit scalability. This paper reviews the properties and design trade-offs of state-of-the-art PFM imagers and introduces a novel DROIC architecture with off-pixel counting. By relocating the counter logic and using an event-based interface, the proposed approach is suited for next-generation PFM DROICs with less than 10 μm pixel pitch.