A Partially Pixel-Parallel DROIC for MWIR Imagers With Columnwise Residue Quantization

This paper presents a partially pixel-parallel digital readout integrated circuit that overcomes the limitation of achievable digital resolution seen in megapixel-format infrared digital focal plane arrays (IR-DFPAs). It incorporates 5-bit in-pixel pulse frequency modulation (PFM)-based charge-to-digital converters and 10-bit successive approximation register (SAR) column analog-to-digital converters (ADCs). To increase the readout resolution, the residue charge at the end of the integration phase is fine converted to a 10-bit digital word and combined with the in-pixel data. This method is a compromise between fully pixel-parallel and fully column-parallel conversion approaches and favors the ever-growing trend toward small pitch FPAs by allowing a compact pixel size. A prototype consisting of a $32\times32$ array of pixels, 32 column SAR ADCs, and a timing controller block is designed and fabricated in a 90-nm bulk CMOS process. A modified version of the conventional PFM-based pixel is designed to help hold the residue charge. The design is targeted at medium-wave IR-DFPAs with frame rates up to 400 Hz. A signal-to-noise ratio of 79 dB is achieved with a full-well capacity of 2.4 Me-.