Offset-Free Offset Correction for Active Pixel Sensors

Abstract

Introduction The output signal of active pixels has a non-uniformity caused by technological variations of the electronic components composing the pixel. If no precautions are taken, this non-uniformity (fixed pattern noise or FPN) is seen as a "snow-like" shade over the image. We describe a method to improve the reading of active pixels, so that this FPN is cancelled, with as final result a better cosmetic quality of the image. Several solutions to this problem have been proposed in literature. We described a technique based upon shifting the threshold voltage of the source follower transistor in the pixel [1] and a correction technique with a dedicated co-processor [2]. The solution proposed here is a correlated double sampling technique, which is a widespread technique to tackle this problem for integrating pixels [3, 4, 5]. For this purpose, it must be possible to bring the active pixels in a reference state that corresponds to a known amount of collected l