Column A/D Conversion in Image Sensors to Prevent Shading
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Solution Overview
Problem
In solid-state image pickup apparatuses, the use of column A/D conversion circuits with a fixed reference voltage leads to potential gradients in the reference voltage line, causing current differences between columns, which results in shading effects when converting analog signals to digital signals, especially as the number of columns and line length increase.
Innovation Solution
The apparatus employs a constant current supply unit that integrates and distributes a constant current to each A/D conversion unit via an input capacitor, ensuring uniform current flow and minimizing potential gradients across columns, thereby stabilizing the reference signal for accurate A/D conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a fixed reference voltage is supplied to column A/D conversion circuits, then the conversion speed is improved, but potential gradients occur in the reference voltage line causing current differences between columns
Solution Approach 1:
The patent changes the parameter of the reference signal from a fixed voltage to a time-varying voltage that decreases monotonically. This parameter change allows the reference signal to compensate for the potential gradients in the reference voltage line, ensuring that all columns receive equivalent effective reference signals despite the voltage drops, thereby maintaining uniform conversion characteristics across columns while preserving high conversion speed.
Solution Approach 2:
The patent introduces a dynamic reference voltage signal that varies over time rather than using a static fixed voltage. The reference voltage decreases monotonically during the conversion period, creating a dynamic compensation mechanism that counteracts the static potential gradients in the reference voltage line, thus resolving the contradiction between speed and uniformity.
2Area of stationary object
If the number of columns and line length are increased, then the coverage area is improved, but shading effects become more pronounced due to potential gradients
Solution Approach 1:
The patent applies parameter changes to the reference voltage signal, transforming it from a fixed value to a time-varying signal that monotonically decreases. This parameter transformation enables the reference signal to compensate for the potential gradients that become more pronounced with increased line length and number of columns, thereby eliminating shading effects while allowing the pixel array to cover larger areas.
3Speed
If a constant current passes through resistor R in each A/D conversion circuit, then conversion speed is enhanced, but voltage differences across the resistor cause current variations between columns
Solution Approach 1:
The patent changes the parameter of the reference signal from a fixed voltage level to a time-varying voltage that monotonically decreases. This parameter change ensures that despite the voltage drops across the reference voltage line and resistor R, all columns experience equivalent effective voltage conditions, maintaining consistent current flow and reliable operation across all columns while preserving enhanced conversion speed.
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 solution effectively suppresses shading effects along rows by providing a uniform reference signal to comparators, ensuring consistent A/D conversion performance across columns and reducing the impact of potential gradients on the reference signal supply line.
Implementation Method 1
The integration unit includes an input capacitor configured to receive the constant current from one terminal thereof
Data Source
AI summary
An apparatus includes a pixel array in which pixels for outputting an analog signal are arranged in a matrix, vertical output lines each of which is connected to pixels in a same column, A/D conversion units, which are individually connected to the vertical output lines, for converting the analog signal into a digital signal, and a constant current supply unit for supplying a constant current to the A/D conversion units. Each of the A/D conversion units includes an integration unit for integrating the constant current, a comparison unit for comparing the integrated constant current with the analog signal and outputting a comparison signal, and a digital signal storage unit for storing a digital signal corresponding to the comparison signal. The integration unit includes an input capacitor for receiving the constant current. The comparison unit is connected to the constant current supply unit via the input capacitor.