HDR Pixel Circuit Calibration for Image Sensing Precision
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Solution Overview
Problem
Pixel-to-pixel variation of photodiodes in MOS imager circuits leads to higher noise levels and lower precision in image sensing due to varying calibration full-well capacities, even when using nonlinear voltage-to-light transfer functions for high dynamic range imaging.
Innovation Solution
A method is introduced to calibrate the calibration full-well capacities of each pixel circuit by over-charging and dissipating charges on the photodiode and floating diffusion node, then transferring these charges to sense the voltage on the floating diffusion node, generating a calibration signal to account for pixel-to-pixel variations and achieve precise image sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If nonlinear voltage-to-light transfer function is used for high dynamic range imaging, then dynamic range is extended, but pixel-to-pixel variation causes higher noise levels and lower precision
Solution Approach 1:
The patent applies preliminary action by performing calibration before actual imaging. The method includes: (1) setting the transfer signal to a first predetermined voltage level to establish a calibration full-well capacity; (2) over-charging both the photodiode and floating diffusion node; (3) dissipating charges to achieve substantially equal charges on both nodes; (4) transferring charges from photodiode to floating diffusion node; and (5) sensing the voltage to generate a calibration signal. This pre-calibration process compensates for pixel-to-pixel variations, reducing noise and improving precision while maintaining the extended dynamic range enabled by the nonlinear transfer function.
2Measurement precision
If calibration is performed to reduce pixel-to-pixel variation, then precision is improved, but additional calibration steps increase process complexity
Solution Approach 1:
The patent applies self-service by using the pixel circuit's own components to perform calibration. The calibration process utilizes the existing photodiode, transfer transistor, and floating diffusion node without requiring external calibration equipment. The method leverages the natural charge transfer mechanism and voltage sensing capability of the pixel circuit itself to generate calibration signals that compensate for its own variations, thereby improving precision while avoiding additional complex external calibration systems.
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 method enhances the precision and reduces noise in image sensing by accurately calibrating pixel circuits, enabling high dynamic range imaging with improved precision and reduced noise levels.
Implementation Method 1
a photodiode PD, configured to operably accumulate charges generated by a photocurrent of the photodiode during an integration period
Implementation Method 2
a transfer transistor MTG, controlled by a transfer signal TG... When the transfer signal TG is set to a high level (Von), all the charges on the photodiode PD is transferred to the floating diffusion node FD through the transfer transistor MTG
Implementation Method 3
a buffer transistor MSF, configured to operably sense a voltage Vfd on the floating diffusion node FD
Data Source
AI summary
A method of operating an HDR pixel circuit includes: establishing a calibration full-well capacity of a photodiode according to a first predetermined voltage level; over-charging both the photodiode and a floating diffusion node; dissipating the charges of the floating diffusion node and the charges on the photodiode so that the charges on the photodiode are substantially equal to the calibration full-well capacity; transferring the charges on the photodiode to the floating diffusion node; and sensing a voltage on the floating diffusion node to generate a calibration signal related to the calibration full-well capacity.