CMOS Image Sensor Comparator Using Fixed Reference Voltage
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Solution Overview
Problem
CMOS image sensors face challenges in maintaining image quality due to varying correlated double sampling characteristics with input common mode voltage, leading to noise and reduced linearity, especially with single-ended AC-coupled comparators, and increased area requirements with differential AC-coupled comparators.
Innovation Solution
A comparison device using one small sampling capacitor for the input terminal and a fixed reference voltage, which includes a comparator, a first sampling capacitor between the input terminal and the comparator, a sampling switch, and a second sampling capacitor between the ground and the comparator's negative input terminal, to stabilize the input common mode voltage and improve linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a single-ended AC-coupled comparator with one sampling capacitor is used, then the area is reduced, but the input common mode voltage varies causing noise and reduced linearity
Solution Approach 1:
The patent implements a feedback mechanism where the comparator output is fed back through a sampling capacitor to the negative input terminal. This feedback structure stabilizes the input common mode voltage by automatically adjusting for voltage variations, thereby maintaining linearity while using only one sampling capacitor at the input terminal.
Solution Approach 2:
The patent changes the operating parameters by using a fixed reference voltage instead of allowing the input common mode voltage to vary with ramp signal swing. This parameter stabilization ensures that the comparator operates at a constant common mode voltage level, improving linearity without requiring two sampling capacitors.
2Object-affected harmful factors
If a differential AC-coupled comparator with feedback structure is used, then ramp noise is reduced, but the area increases due to two input sampling capacitors
Solution Approach 1:
The patent uses a feedback structure where the comparator output connects to the negative input terminal through a sampling capacitor. This feedback mechanism captures and compensates for the input common mode voltage, effectively reducing ramp noise while requiring only one input sampling capacitor instead of two.
Solution Approach 2:
The patent combines the functions of multiple sampling capacitors into a single input sampling capacitor by using the feedback structure. The feedback path effectively performs the noise reduction function that would otherwise require a second capacitor, merging multiple functions into a more compact configuration.
3Adaptability or versatility
If the swing voltage of ramp signal is changed according to analog gain, then the analog gain is adjusted, but the input common mode voltage changes serving as noise source
Solution Approach 1:
The patent stabilizes the input common mode voltage parameter at a fixed reference level while allowing the ramp signal swing to vary according to analog gain requirements. By decoupling the common mode voltage from the ramp signal amplitude variations, the system achieves gain adjustability without introducing noise from common mode voltage fluctuations.
Data Source
AI summary
Provided are a comparison device capable of achieving a small area by using one small sampling capacitor for an input terminal and improving linearity by using a fixed reference voltage and a CMOS image sensor using the same. The comparison device may include a comparator configured to compare a pixel signal inputted through a positive input terminal with a ramp signal, a first sampling capacitor configured to be provided between an input terminal of the ramp signal and the positive input terminal of the comparator, a sampling switch configured to be provided between an output terminal of the comparator and a negative input terminal of the comparator, and a second sampling capacitor configured to be provided between a ground terminal and the negative input terminal of the comparator.