Improvement in Random Noise for Pixel-Parallel Single-Slope ADC with Consideration of Flicker Noise Effect

We propose and demonstrate a low-random-noise (RN) design for pixel-parallel single-slope ADCs (SS-ADCs), achieving 2.2 e-rms in a 3.24 µm pixel. In this paper, we discuss AC-based RN estimation with respect to the comparator bias current and a bandwidth-limiting capacitor in digital-pixel sensors (DPSs). RN is composed of thermal noise (TN) and flicker noise (FN), where FN can be a major contributor in DPSs because of its area limitation. We express the concise equation to estimate the FN/TN ratio, in which the FN characteristic is modulated by the correlated double sampling (CDS) operation. We also study the effective RN bandwidth, which increases due to the ramp slope transient effect and introduces a noise bandwidth (NBW) coefficient, to estimate the effective NBW. This study provides insights into the area arrangement of the small-pixel DPS design. A high-gain single-ended comparator is introduced to realize an area-efficient DPS without digital CDS (D-CDS). Noise analysis of its pixel design shows that FN becomes the main contributor, and further RN improvement by limiting NBW or D-CDS is not promising under these conditions.