Dual Conversion Gain Pixel Voltage Stress Reduction
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Solution Overview
Problem
Conventional imaging systems face issues with image pixels in a given row being forced into high or low gain modes, leading to excessive noise or over-saturation, and voltage stress due to boosted control signals, which limits the lifetime of image sensors.
Innovation Solution
Implementing per-column gain adjustments using dual conversion gain transistors that can be controlled without boosting control signal voltages, allowing each pixel to operate in either high or low gain modes based on image signal characteristics, reducing voltage stress and improving image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If uniform gain mode control is applied to all pixels in a pixel row, then the control circuitry is simple, but some pixels generate excessively noisy or over-saturated image signals
Solution Approach 1:
The patent divides the pixel array into multiple independently controllable groups (e.g., by column or by region within a row). Each group can have its gain mode independently adjusted based on local scene brightness, preventing over-saturation in bright areas and noise in dark areas while keeping the overall control structure manageable.
Solution Approach 2:
The patent implements local gain adjustment where different portions of the pixel array can operate in different gain modes simultaneously. This allows each local region to be optimized for its specific lighting conditions, with dark regions using high gain and bright regions using low gain, thereby improving overall image signal quality.
2Ease of operation
If boosted gain adjustment control signals are provided to place pixels in high or low gain modes, then gain adjustment is effective, but voltage stress is induced on the image sensor
Solution Approach 1:
The patent changes the voltage parameter of the gain adjustment control signals from boosted levels to non-boosted levels that are within the normal operating voltage range of the pixel circuitry. This is achieved by redesigning the pixel circuit to respond to standard voltage levels, thereby maintaining gain adjustment effectiveness while eliminating voltage stress and improving sensor reliability.
3Manufacturing precision
If per-pixel gain adjustment is implemented, then image quality is improved, but the device complexity increases
Solution Approach 1:
The patent merges the gain adjustment control with existing pixel circuitry elements, such as combining the gain control transistor with the readout circuitry or integrating control functions into existing pixel structures. This reduces the need for separate dedicated gain control components, thereby improving image quality while minimizing increases in device complexity.
Data Source
AI summary
An image sensor may include an array of pixels arranged in rows and columns. Each pixel may include a floating diffusion node, a capacitor, a dual conversion gain (DCG) transistor having a gate terminal coupled to the floating diffusion, a source terminal, and a drain terminal coupled to the floating diffusion through the capacitor. Column readout circuitry may provide per-column control signals to the source terminal of the DCG transistor in the pixels of a selected row to place the pixels into a low conversion gain mode by turning the DCG transistor on and into a high conversion gain mode by turning the DCG transistor off. In this way, the readout circuitry may provide per-column conversion gains for each row without boosting DCG control signals to magnitudes greater than the pixel supply voltage, thereby reducing voltage stress on the pixel array and improving lifetime of the image sensor.