Comparator Output Swing Control Circuit for CMOS Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensors face issues with image resolution reduction due to banding noise and kick-back errors caused by changes in the operation regions of transistors during analog-to-digital conversion, which also lead to increased power consumption and chip area requirements.
Innovation Solution
A comparator design with an output swing control circuit that maintains an output voltage at or above a preset level, preventing changes in the operation region of transistors and minimizing the influence of output voltage changes on parasitic capacitors, thereby reducing banding noise and kick-back errors, without the need for buffers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional comparator design is used without output swing control, then circuit complexity is reduced, but banding noise and kick-back errors increase due to transistor operation region changes
Solution Approach 1:
The patent implements dynamic control of the output swing range through an output swing control circuit that adjusts the common voltage of transistors based on the comparison result. This dynamic adjustment prevents transistor operation region changes that cause banding noise and kick-back errors, while adapting to different signal conditions to maintain low complexity.
Solution Approach 2:
The patent changes the voltage parameter (common voltage) dynamically to maintain transistors in the saturation region. By adjusting the common voltage level based on comparison outcomes, the system prevents operation region transitions that generate noise and errors, resolving the contradiction between simplicity and noise reduction.
2Object-affected harmful factors
If buffers are added to prevent operation region changes, then banding noise and kick-back errors are reduced, but chip area increases
Solution Approach 1:
The patent extracts and eliminates the need for separate buffer circuits by integrating the operation region control function directly into the comparator core through the output swing control circuit. This integration achieves noise reduction without the additional chip area that buffers would require.
Solution Approach 2:
The output swing control circuit performs multiple functions: it controls the common voltage to prevent operation region changes, adjusts output swing range, and maintains transistor saturation. This multi-functionality replaces what would otherwise require separate buffer circuits, reducing chip area while maintaining noise reduction benefits.
3Stability of the object's composition
If output swing range is increased to improve signal margin, then operation region stability is improved, but power consumption increases
Solution Approach 1:
The patent implements dynamic adjustment of the output swing range rather than using a fixed large swing. The output swing control circuit adapts the voltage levels based on comparison results, maintaining sufficient signal margin for stability while minimizing the voltage swing amplitude to reduce power consumption.
Solution Approach 2:
The patent dynamically changes voltage parameters (common voltage and output swing levels) to achieve the minimum necessary swing for stable operation. By optimizing these parameters rather than using excessive voltage ranges, the system maintains operation region stability while minimizing power consumption.
Data Source
AI summary
A comparator is provided to comprise a comparison circuit including a first transistor configured to receive a ramp signal and a second transistor configured to receive a pixel signal, the comparison circuit configured to compare the ramp signal and the pixel signal and output a comparison signal at an output node, and an output swing control circuit including a third transistor coupled to the first transistor and the second transistor, and the output swing control circuit including a current path to decrease an amount of current flowing through the second transistor.