On analog comparators for CMOS digital pixel applications. A comparative study

Abstract

Abstract. Voltage comparator is the only -apart from the light-to-voltage converter -analog component in the digital CMOS pixel. In this work, the influence of the analog comparator nonidealities on the performance of the digital pixel has been investigated. In particular, two versions of the digital pixel have been designed in 0.35 µm CMOS technology, each using a different type of analog comparator. The properties of both versions have been compared. The first pixel utilizes a differential comparator with the increased size and improved electrical performance. The second structure is based on a very simple non-differential comparator with a reduced size and degraded performance. Theoretical analysis of the comparator nonideality effect on the quality of the image obtained from the digital pixel matrix as well as simulation results are provided.Key words: CMOS image sensor, CMOS digital pixel, analog comparator, fixed pattern noise (FPN). On analog comparators for CMOS digital pixel applications.A comparative study W. JENDERNALIK* Faculty of Electronics Telecommunications and Informatics, Gdańsk University of Technology, 11/12 Narutowicza St., 80-952 Gdańsk, Poland pixels containing a photosensor and a complete A/D converter are larger than 20 µm × 20 µm. This results in degradation of the fill factor, increase of the matrix area, and the increase of the integrated image sensor price. For this reason, development of A/D converter solutions that can be implemented on a small pixel while maintaining sufficiently large bit resolution and high conversion quality becomes of primary importance. One of the most important components of the A/D converter is a CMOS analog voltage comparator. Such a comparator usually consists of a conventional MOS differential pair with a current mirror as an active load, an amplifier stage implemented as a common-source configuration, as well as one or two CMOS inverters which work as an additional amplification stage. The comparator footprint may contribute to as much as 20 to 30 percent of the entire pixel area [7]. In order to reduce the comparator size, one needs to reduce the sizes of its transistors, which leads to degrading the electrical performance of the circuit. Smaller transistor size means larger technological spread of their parameters and the increased offset voltage of the comparator. Moreover, the 1/f noise level is also increased.In this work, we investigate a possibility of applying -in a digital pixel -a simple non-differential analog comparator consisting of four CMOS transistors. Such a comparator is characterized by worse electrical performance compared to a conventional differential comparator; however, it features a considerably smaller footprint. In order to compare the influence of the analog comparator performance on the image quality, two versions of a digital pixel realized in 0.35 µm CMOS technology have been designed. The pixels are only different with respect to the analog comparator. Simulation studies have been performed for the two matrices of the siz...

An Ultra-Low-Energy Analog Comparator for A/D Converters in CMOS Image Sensors

“…Accurate analysis of the mismatch will not be carried out here. Examples of such analysis for analog comparators can be found in [6,14]. The estimated value of V os based on the simulations is below ±10 mV (1 sigma) for all comparators from Fig.…”