Autozeroing ADC Offset Compensation for Image Sensor Uniformity
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-speed CMOS image sensors face column-to-column nonuniformity due to comparator input offset, which is not completely removed by existing calibration methods, and this nonuniformity results in artifacts in the sampled image, with calibration circuits occupying a significant portion of the circuit area.
Innovation Solution
Implementing an autozeroing amplifier section and comparator section in the ADC circuit to amplify and stabilize input signals, reducing the effect of comparator input offset through multiple amplifier stages and offset reduction operations, thereby compensating for offset values at capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing calibration methods are used to compensate comparator offset, then some offset compensation is achieved, but column-to-column nonuniformity remains and artifacts appear in the sampled image
Solution Approach 1:
The patent performs offset compensation in advance by sampling the offset value into a calibration capacitor before the actual signal conversion. The autozeroing amplifier samples the comparator offset during a calibration phase, storing it in a capacitor. During signal conversion, this pre-sampled offset is subtracted from the input signal, effectively compensating for the comparator offset before it can cause measurement errors or image artifacts.
2Measurement precision
If calibration circuits are extended to improve offset compensation, then measurement precision improves, but circuit area increases significantly
Solution Approach 1:
The patent combines the calibration function with the existing autozeroing amplifier circuit that is already present in the ADC. The autozeroing amplifier performs dual functions: (1) its traditional function of reducing amplifier offset, and (2) sampling the comparator offset into a calibration capacitor. This merging eliminates the need for separate calibration circuits, achieving accurate offset compensation while minimizing additional circuit area.
Solution Approach 2:
The autozeroing amplifier is designed to perform multiple functions: it serves as both the traditional autozeroing amplifier for reducing amplifier offset and as an offset sampling device for capturing comparator offset. This multi-functionality allows the same circuit hardware to accomplish what would traditionally require separate dedicated calibration circuits, thereby reducing overall circuit area while maintaining measurement precision.
Data Source
AI summary
An image sensor may be improved by using ADCs that compensate for the effect of comparator input offset on comparator decisions. Offset compensation may be implemented in an ADC by using an amplifier section between the input of the ADC and a comparator section of the ADC to amplify the signals supplied to the comparator inputs and thereby reduce the effect of comparator offset on the comparator decision. The comparator section may be an autozeroing comparator section that is capable of performing an offset reduction operation to store offset compensation values at capacitors provided at its inputs. The amplifier section may be an autozeroing amplifier section having one or more amplifier stages that are capable of performing an offset reduction operation to store offset compensation values at capacitors provided at their inputs. Offset compensation may also be implemented using an autozeroing comparator section without a preceding amplifier section. Related methods of operation cause the circuits to perform the offset reduction operations.