Feedthrough-Compensated CMOS Image Sensor Readout
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Solution Overview
Problem
CMOS image sensors face challenges in low-light performance due to increasing readout noise as pixel geometries shrink, leading to signal levels being dominated by readout noise across a wider illumination spectrum, especially in office lighting conditions.
Innovation Solution
The implementation of feedthrough compensation signals to null control-signal feedthrough to the floating diffusion node in CMOS image sensors, allowing for a shortened correlated double-sampling time and reduced pixel readout noise, enabling sub-electron readout and novel architectures like correlated photon counting without avalanche sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pixel geometries are shrunk to increase resolution, then spatial resolution is improved, but readout noise increases due to second-order sampling inaccuracies and reduced electron capture
Solution Approach 1:
The patent applies preliminary anti-action by asserting feedthrough compensation signals before and during the readout process to preemptively counteract the capacitive coupling effects. These compensation signals are generated based on the timing and amplitude of the transfer gate and reset gate control signals, effectively canceling out the feedthrough noise before it degrades the pixel signal, thereby maintaining low readout noise even in shrunk pixel geometries
2Object-affected harmful factors
If conventional CDS readout is used, then readout noise is reduced through correlation, but temporal noise increases due to prolonged CDS interval caused by feedthrough settling time
Solution Approach 1:
The patent converts the harmful feedthrough effect into a beneficial compensation mechanism. By deliberately generating feedthrough compensation signals that mirror the transfer gate and reset gate control signals, the system transforms the parasitic capacitive coupling into a controlled compensation action. This allows the CDS interval to be significantly shortened while maintaining noise reduction performance, as the compensation signals actively counteract the feedthrough during the correlated sampling process
3Measurement precision
If pixel size is reduced, then resolution is improved, but electron capture ability decreases leading to lower signal levels dominated by readout noise
Solution Approach 1:
The patent introduces feedthrough compensation signals as an intermediary mechanism between the pixel photodetector and the readout circuitry. These compensation signals act as a mediator that isolates the weak pixel signal from the dominant readout noise by actively canceling the capacitive feedthrough effects. This intermediary compensation layer enables small pixels to maintain adequate signal levels despite reduced electron capture ability, as the compensation signals subtract the feedthrough noise component from the total readout signal
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces pixel readout noise, allowing for nanoseconds-order CDS time and sub-electron pixel readout, enhancing low-light performance and dynamic range without compromising noise levels.
Implementation Method 1
capacitive feedthrough from control signal lines to the floating diffusion node
Data Source
Figure 1A~1B
Figure 2~3
Figure 4~7
AI summary
A control pulse is generated a first control signal line coupled to a transfer gate of a pixel to enable photocharge accumulated within a photosensitive element of the pixel to be transferred to a floating diffusion node, the first control signal line having a capacitive coupling to the floating diffusion node. A feedthrough compensation pulse is generated on a second signal line of the pixel array that also has a capacitive coupling to the floating diffusion node. The feedthrough compensation pulse is generated with a pulse polarity opposite the pulse polarity of the control pulse and is timed to coincide with the control pulse such that capacitive feedthrough of the control pulse to the floating diffusion node is reduced.