Single-Slope ADC with Embedded Convolution Filter for Global-Shutter CMOS Image Sensors

Abstract

This paper presents an analog-to-digital converter (ADC) suitable for acquisition and processing of images in the global-shutter mode at the pixel level. The ADC consists of an analog comparator, a multi-directional shift register for the comparator states, and a 16-bit reversible binary counter with programmable step size. It works in the traditional single-slope mode. The novelty is that during each step of the reference ramp, neighboring pixels can exchange status information. During the conversion, the direction and step size of the counter are set globally to realize the corresponding coefficient of a convolution kernel. This technique does not slow down the conversion when used for small kernels (3W3) and does not significantly increase sensor noise. Convolution windows of arbitrary size can be implemented. The concept was verified in an experimental 64W64 imaging array implemented in 180 nm CMOS technology.