CMOS Image Sensor Dual-Capacitor Correlated Readout Noise Reduction
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Solution Overview
Problem
Traditional CMOS image sensors with global shutters suffer from noise due to uncorrelated read-outs of image and reset signals, leading to decreased signal-noise ratio (SNR) when capturing high-speed moving objects.
Innovation Solution
The proposed imaging apparatus and method incorporate a pixel array with capacitors for storing reset and pixel signals, allowing for correlated double sampling by reading out the signals from these capacitors, which are stored after the reset signal is stored, and using a column circuit with an amplifier and auto-zeroing switch to eliminate noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a global shutter is used to capture high-speed moving objects without distortion, then image distortion is eliminated, but noise increases due to uncorrelated read-outs of image and reset signals
Solution Approach 1:
The reset signal is stored in the first capacitor before the pixel signal is transferred and stored in the second capacitor. This preliminary storage of the reset signal enables subsequent correlated read-out operations, where both reset and pixel signals are read from the same holding capacitor through the same amplifier, ensuring that noise characteristics are identical and can be fully subtracted during the correlation process.
Solution Approach 2:
A dual-capacitor structure (first capacitor Crst and second capacitor Csig) is introduced as an intermediary mechanism between the pixel array and the read-out amplifier. The first capacitor holds the reset signal voltage, while the second capacitor holds the pixel signal voltage. Both capacitors connect to the same amplifier input through switches, enabling correlated read-out where the amplifier processes both signals under identical conditions, thus eliminating noise through correlation.
2Device complexity
If traditional rolling shutter read-out is used, then device complexity is reduced, but image distortion occurs on high-speed moving objects
Solution Approach 1:
The pixel array is segmented into multiple independent pixel units, each equipped with its own dual-capacitor structure (Crst and Csig) for storing reset and pixel signals respectively. This segmentation allows parallel processing of multiple pixels while maintaining the global shutter function, reducing the need for complex sequential read-out mechanisms while preventing image distortion.
3Speed
If uncorrelated read-out of reset and pixel signals is used, then read-out speed is improved, but signal-noise ratio decreases
Solution Approach 1:
The read-out circuit employs a feedback mechanism where the amplifier processes both the reset signal (from first capacitor Crst) and pixel signal (from second capacitor Csig) through the same signal path. The correlated read-out ensures that noise introduced during read-out is identical for both signals, and this correlated noise is completely eliminated when the signals are subtracted, thereby maintaining high signal-noise ratio while enabling fast read-out.
Data Source
AI summary
The present invention relates to an imaging apparatus, including a pixel array comprising a plurality of pixels arranged in rows and columns, wherein at least one of the pixels comprises: a first capacitor configured to store a reset signal, and a second capacitor configured to store a pixel signal; a plurality of column circuits, wherein at least one of the column circuits reads the reset signal from the first capacitor, reads the pixel signal from the second capacitor, and generates difference between the reset signal and the pixel signal, wherein the pixel is configured to store the pixel signal to the second capacitor after the reset signal is stored to the first capacitor.