Noise Analysis of Pixel-Parallel ADC

Abstract

This study discusses the noise of a pixel-parallel ADC. An image sensor using a pixel-parallel ADC was designed to operate on subthreshold current, usually less than 1 μA per
pixel, constrained by the increase in the number of simultaneous operations of the comparator. Therefore, the mechanism of noise generation in this imager is dominated not by saturation but by diffusion current. We analyzed the design parameters having sensitivity to noise through a circuit model equation. The analysis and prototype measurement results were applied to the transient noise simulation performed using 90 nm-pixel/40 nmlogic model parameters. It was found that the effective design parameters are the operation current of the comparator, slope of the DAC, and load capacitance of the 2nd node. Finally, leveraging the feedback from this study, we designed a chip with stacked 25.2 Mpixels of 5.94 µm2, achieving 167 µVrms at 0 dBanalog gain and 120 fps operation with a power consumption of 1545 mW at 14 bit resolution

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