Dual Conversion Gain Image Sensor Anti-Eclipse Circuitry
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Solution Overview
Problem
The eclipse phenomenon in image sensors causes erroneous reset signals due to strong light exposure, leading to dark pixels appearing bright, which is not effectively addressed by existing technologies.
Innovation Solution
The implementation of anti-eclipse circuitry that clamps the pixel output voltage during reset sampling to a minimum value, preventing voltage drops in the floating diffusion region and mitigating column fixed pattern noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pixels are exposed to strong light during reset operations, then the photodiode generates charge that should brighten the pixel, but the floating diffusion leaks causing erroneous reset signals that make the pixel appear dark
Solution Approach 1:
An anti-eclipse circuit is introduced as an intermediary component between the pixel and the readout circuitry. This circuit includes a switch connected in parallel with the pixel output that is selectively activated during reset operations to clamp the output voltage and prevent the floating diffusion leakage from corrupting the reset signal measurement.
Solution Approach 2:
The anti-eclipse circuit applies a preliminary corrective action by clamping the pixel output voltage to a minimum level during reset operations, before the erroneous reset signal can be sampled. This preemptive measure prevents the voltage drop caused by floating diffusion leakage from affecting the reset signal accuracy.
2Reliability
If anti-eclipse circuitry is added to correct eclipse phenomenon, then reset signal accuracy improves, but device complexity increases
Solution Approach 1:
The anti-eclipse circuit is merged with the existing pixel structure by sharing the pixel output node and integrating the switch control with the existing reset gate timing. The switch is controlled by the same reset signal that already controls the pixel reset operation, eliminating the need for separate control circuitry.
Solution Approach 2:
The anti-eclipse switch serves multiple functions: it clamps the output voltage during reset operations to prevent eclipse effects, and can be integrated with the existing reset gate to provide both pixel reset and anti-eclipse protection simultaneously through a single control signal.
3Measurement precision
If floating diffusion leaks under strong illumination, then reset signal voltage drops below desired level, but adding circuitry to prevent this increases manufacturing complexity
Solution Approach 1:
The anti-eclipse circuit utilizes the existing reset signal and pixel output node to provide self-service protection. The switch is controlled by the same reset signal that resets the pixel, and the circuit automatically activates when needed without requiring additional control signals or external intervention.
Solution Approach 2:
The circuit changes the electrical parameter state by switching the anti-eclipse transistor on during reset operations to clamp the output voltage at a minimum level, and off during normal operation to allow full dynamic range. This parameter change is controlled by the reset signal timing rather than requiring precise voltage level control during manufacturing.
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
Ensures accurate pixel signal computation by maintaining a stable reset charge level, reducing the occurrence of dark pixels in bright conditions and eliminating column fixed pattern noise without interfering with non-eclipse operations.
Implementation Method 1
Each image pixel in the array includes a photodiode that is coupled to a floating diffusion region via a transfer gate
Data Source
AI summary
An image sensor may include an array of dual conversion gain image pixels arranged in rows and columns. The image pixels arranged along the same column may be coupled to a column line. The column line may be coupled to anti-eclipse control circuitry. In one suitable arrangement, the anti-eclipse control circuitry may include a comparator that compares the output signal on the column line to an anti-eclipse bias voltage. If, during a reset sampling period, the output signal on the column line is less than the anti-eclipse bias voltage, a current source may be used to charge the bottom plate of a dual conversion gate capacitor in the selected image pixel to help restore the voltage of the floating diffusion node in the selected pixel.