CMOS Image Sensor Ground Line Noise Suppression
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Solution Overview
Problem
Photoelectric conversion devices, such as CMOS image sensors, face challenges in suppressing magnetic noise superimposed on the ground line and horizontal stripe noise due to the lack of consideration for magnetic noise in existing AD converter circuit designs using ramp signals.
Innovation Solution
A photoelectric conversion device is designed with a semiconductor substrate, pixel well regions, and a reference signal generation circuit that includes comparator units with capacitors connected between the reference signal and the pixel ground line, effectively reducing magnetic noise while suppressing horizontal stripe noise by adjusting the amplitude of the ramp signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a common ground line is used for pixels and reference signal generation circuit, then device complexity is reduced, but magnetic noise affects pixel signals
Solution Approach 1:
The ground line is segmented into a pixel ground line for pixels and a circuit ground line for the reference signal generation circuit, allowing separate grounding paths that prevent magnetic noise from the circuit ground from affecting pixel signals while maintaining structural simplicity
Solution Approach 2:
A ground isolation circuit is introduced as an intermediary between the pixel ground line and circuit ground line, consisting of a first ground line connected to the pixel ground, a second ground line connected to the circuit ground, and a switch that selectively connects or disconnects these ground lines based on operational mode
2Measurement precision
If ramp signal amplitude is increased to improve signal-to-noise ratio, then pixel signal quality improves, but horizontal stripe noise increases
Solution Approach 1:
The ramp signal amplitude is made dynamic rather than fixed, allowing the amplitude to be adjusted based on lighting conditions and noise characteristics, thereby optimizing the balance between signal-to-noise ratio and horizontal stripe noise suppression
Solution Approach 2:
The amplitude parameter of the ramp signal is changed adaptively according to different operating conditions, enabling the system to maintain high measurement precision while suppressing horizontal stripe noise by selecting appropriate amplitude levels
Data Source
AI summary
An embodiment includes: a semiconductor substrate including a pixel well region and a peripheral well region; a pixel ground line arranged above the pixel well region; a pixel well contact between the pixel ground line and the pixel well region; pixels arranged to form columns in the pixel well region; a reference signal generation circuit arranged in the peripheral well region; and comparator units arranged in the peripheral well region, provided to respective columns, and each configured to receive the pixel signal from the pixels on a corresponding column and the reference signal. Each comparator unit includes a comparator having a first input node that receives the pixel signal and a second input node that receives the reference signal, a first capacitor unit between the reference signal generation circuit and the second input node, and a second capacitor unit between the second input node and the pixel ground line.