LOFIC Image Sensor Readout Using Column Feedback Amplifier
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Solution Overview
Problem
LOFIC-based image sensors face challenges in precisely reading photodiode voltage due to the reset signal interfering with the charge measurement, leading to inaccurate analog-to-digital conversion and voltage offset issues.
Innovation Solution
The implementation of a column-level feedback amplifier to synchronize the voltage levels of dummy and active pixels, ensuring accurate voltage intersection and reducing thermal noise, thereby improving the readout process and ADC conversion accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a reset signal is applied to the photodiode prior to voltage readout, then the voltage measurement range is improved, but voltage offset and measurement precision deteriorate
Solution Approach 1:
The patent segments the readout process into distinct phases: an integration phase where the photodiode accumulates charge, and a readout phase where the voltage is measured. The reset signal is applied at the boundary between these phases rather than continuously, allowing the system to maintain both measurement range and precision by separating the functions of charge accumulation and voltage measurement.
Solution Approach 2:
The patent applies preliminary actions by pre-charging the photodiode to a specific voltage level before the readout operation. This preliminary charging establishes a known reference state that enables accurate voltage measurements while maintaining the ability to measure across a wide dynamic range. The reset signal is timed to occur just before readout, preparing the photodiode in advance for accurate measurement.
2Manufacturing precision
If device geometries are scaled down to increase integration density, then device integration density is improved, but voltage offset and measurement accuracy worsen
Solution Approach 1:
The patent changes the timing parameters of the reset signal application based on the scaled device geometry. In smaller, more densely integrated devices, the reset signal is applied at precisely controlled moments during the integration cycle rather than continuously. This parameter change allows the system to maintain measurement accuracy despite the reduced physical dimensions and increased integration density, as the timing-based approach compensates for the smaller signal amplitudes present in scaled-down devices.
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 enhances the precision of voltage readout in LOFIC image sensors by eliminating voltage offset and reducing thermal noise, ensuring accurate digital image representation.
Implementation Method 1
The image sensor includes an array of pixels having photosensitive elements (e.g., photodiodes) that absorb a portion of the incident image light and generate image charge upon absorption of the image light
Implementation Method 2
The implementation of a column-level feedback amplifier to synchronize the voltage levels of dummy and active pixels, ensuring accurate voltage intersection and reducing thermal noise
Data Source
AI summary
Switching techniques for fast voltage settling in image sensors are described. In one embodiment, an image sensor includes a plurality of lateral overflow integrating capacitor (LOFIC) pixels arranged in rows and columns of a pixel array. The plurality of pixels includes an active pixel configured for exposure to light, and a dummy pixel at least partially protected from exposure to light. A common bitline (BL) is couplable to the active pixel and the dummy pixel. A comparator (OA1) is coupled to the bitline. The comparator is configured to receive a pixel voltage (Vx) from the active pixel on one input and a ramp voltage (Vy) on another input. Charge accumulated by the active pixel is determined at least in part by an intersection between the ramp voltage and the pixel voltage.