CMOS Image Sensor A/D Converter with Up-Down Counter for Focus Detection
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Solution Overview
Problem
The existing single-slope type A/D converters for CMOS image sensors cannot effectively use techniques that shorten the time required to read out signals from photodiodes sharing a floating diffusion, as they lose reset level information, making it difficult to perform proper difference processing and focus detection.
Innovation Solution
An image sensor design with a comparator and counter that performs multiple operations to convert analog signals from photodiodes into digital signals, allowing for the sharing of a floating diffusion and efficient signal processing by adjusting counter directions for each operation, enabling the subtraction of reset levels and obtaining image signals for focus detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-slope type A/D converter with UP/DOWN counter is used to convert analog pixel signals, then the circuit scale is reduced by using only one counter, but the reset level information is lost when reading out signals from photodiodes sharing a floating diffusion, making it impossible to perform proper difference processing
Solution Approach 1:
The patent applies dynamics by making the counter's counting direction changeable based on the operation phase. The counter can count up during reset level reading and count down during signal reading, or vice versa. This dynamic adaptability allows the single counter to preserve reset level information while performing difference processing, resolving the contradiction between circuit simplicity and information preservation.
Solution Approach 2:
The patent changes the parameter of counting direction (up or down) based on the reading phase. By controlling the counting direction parameter dynamically, the system can maintain reset level information in the counter and perform proper subtraction operations, solving the information loss problem while keeping the single-counter architecture.
2Device complexity
If the counter direction is inverted to subtract reset component from signal component, then only one counter is needed without requiring a subtractor, but the reset level information is lost after the first photodiode reading, preventing proper difference processing for subsequent photodiodes
Solution Approach 1:
The patent makes the counter's operating mode dynamic by allowing it to switch between counting up and counting down based on the current operation phase. This enables the counter to preserve reset level information across multiple photodiode readings while still performing subtraction operations, maintaining both circuit simplicity and reading efficiency.
Solution Approach 2:
The patent performs preliminary action by reading the reset level first and storing it in the counter before reading the signal from the first photodiode. This preliminary storage of reset level information allows subsequent difference processing to be performed correctly without requiring additional subtractor circuits, maintaining efficiency while enabling proper multi-photodiode processing.
3Productivity
If two photodiodes share a floating diffusion to reduce readout time, then the time to read out one reset level is saved, but the existing A/D converter cannot properly process the signals from both photodiodes due to loss of reset level information
Solution Approach 1:
The patent applies dynamics by making the counter's counting direction adaptable to different reading phases. This allows the system to maintain reset level information throughout the rapid sequential reading of multiple photodiodes, ensuring both high readout speed and accurate difference processing for focus detection and image signal generation.
Solution Approach 2:
The patent performs preliminary action by storing the reset level information in the counter before proceeding with signal reading. This preliminary storage enables the fast sequential reading methodology to work correctly, as the reset level is preserved and can be subtracted from each photodiode's signal to maintain measurement precision despite the high-speed readout process.
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 design allows for the efficient conversion and subtraction of signals from multiple photodiodes, reducing the time required for signal output and enabling proper focus detection and image signal generation.
Implementation Method 1
two photodiodes provided below one microlens... configured to receive light from different pupil planes
Data Source
AI summary
An image sensor includes a plurality of pixels arranged in matrix, each pixel including a plurality of photoelectric conversion portions, and analog/digital converters configured to convert analog signals from pixels into digital signals provided by column. The analog/digital converter performs a first operation of converting an analog signal corresponding to the reset level into a digital signal, a second operation of converting analog signals obtained from some portions of the plurality of photoelectric conversion portions into digital signals, and a third operation of converting analog signals obtained from all the plurality of photoelectric conversion portions into digital signals.