A PFM based digital pixel with off-pixel residue measurement for small pitch FPAs

Digital pixels based on pulse frequency modulation (PFM) employ counting techniques to achieve very high charge handling capability compared to their analog counterparts. Moreover, extended counting methods making use of left-over charge (residue) on the integration capacitor help improve the noise performance of these pixels. However, focal plane arrays (FPAs) with small pixel pitch are constrained in terms of pixel area, which makes it difficult to benefit from in-pixel extended counting circuitry. Thus, in this paper, the authors propose a novel approach to measure the residue outside the pixel using an analog-to-digital converter (ADC). A first prototype of the proposed pixel, in the form of a testbed, has been developed which is aimed at medium-wave infrared (MWIR) imaging arrays having small pixel pitch. The prototype is composed of a pixel front end, a 12-bit SAR ADC, a counter, and a comparator. The front-end is a modified version of the conventional design and has been designed and fabricated in 90nm CMOS, whereas off-the-shelf discrete components have been used to implement the ADC, comparator, and counter. Measured signal-to-noise ratio (SNR) at low illumination levels is 55dB.