A PFM-Based Digital Pixel With an Off-Pixel Residue Measurement for Small Pitch FPAs

Digital pixels based on pulse frequency modulation employ counting techniques to achieve a very high charge-handling capability compared to their analog counterparts. Moreover, extended counting methods that make use of leftover charge (residue) on the integration capacitor help to improve the noise performance of these pixels. However, focal plane arrays 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 brief 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. It is aimed at medium-wave infrared imaging arrays that have a small pixel pitch. The prototype is composed of a pixel front end, a 12-bit successive approximation register ADC, a counter, and a comparator. The front end is a modified version of the conventional design and has been designed and fabricated in 90-nm CMOS, whereas off-the-shelf discrete components have been used to implement the ADC, comparator, and counter. A measured signal-to-noise ratio at low illumination levels is 55 dB.