Image Sensor ADC Control for Dark Current and Bit Resolution
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Solution Overview
Problem
Existing image sensors face challenges in achieving high-quality image capture under conditions with increased dark current, particularly during long-time exposure or high-temperature shooting, where the dark current component affects the bit resolution of pixel signals, leading to deteriorated image quality.
Innovation Solution
The image sensor employs a comparator and counter system that adjusts the reference voltage based on shooting conditions and dark current levels, selecting appropriate slope voltages for analog-to-digital conversion to optimize bit resolution and reduce the impact of dark current, allowing for high-speed reading and improved image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If AD conversion is performed with low bit resolution when the amplitude of a signal is large, then the AD conversion time is shortened, but the image quality deteriorates under conditions where dark current increases
Solution Approach 1:
The patent applies dynamics by making the bit resolution adjustable rather than fixed. The control unit dynamically changes the bit resolution of the ADC based on the shooting condition (specifically, whether dark current correction is needed). When dark current increases, the system switches to high bit resolution to maintain measurement precision, and when dark current is low, it uses low bit resolution to shorten conversion time. This dynamic adaptation resolves the contradiction between conversion speed and precision.
Solution Approach 2:
The patent changes the parameter of bit resolution based on the shooting condition. The control unit determines the appropriate bit resolution (first bit resolution for high dark current conditions, second bit resolution for low dark current conditions) and configures the ADC accordingly. This parameter change allows the system to optimize between conversion time and image quality depending on the actual shooting scenario.
2Measurement precision
If the count time by the counter is extended to improve bit resolution, then the measurement precision is improved, but the AD conversion period increases
Solution Approach 1:
The system dynamically adjusts the count time and bit resolution based on shooting conditions. For signals with large amplitudes, the system uses a shorter count time with lower bit resolution to maintain high-speed reading. For signals with small amplitudes where high precision is needed, the system extends the count time to achieve higher bit resolution. This dynamic adjustment resolves the contradiction between conversion period and measurement precision.
Solution Approach 2:
The patent changes the count time parameter according to the amplitude of the pixel signal and shooting condition. The control unit determines the appropriate count time (shorter for large amplitude signals, longer for small amplitude signals requiring high precision) and configures the counter accordingly. This parameter change enables the system to optimize the AD conversion period while maintaining necessary measurement precision.
3Device complexity
If dark current correction is performed after AD conversion, then the processing is simplified, but the image quality deteriorates when dark current amplitude is large
Solution Approach 1:
The patent applies preliminary action by performing dark current correction before AD conversion rather than after. The control unit subtracts the dark current component from the pixel signal in the analog domain before the signal enters the ADC. This preliminary correction ensures that the AD conversion is performed on the corrected signal, maintaining measurement precision even when dark current amplitude is large, while keeping the overall processing relatively simple.
Data Source
AI summary
An image sensor comprising: a pixel portion that outputs a pixel signal; a setting unit that sets a reference voltage in accordance with a shooting condition; a comparator that compares the reference voltage or one of a plurality of comparison voltages having different slopes which change with time and the pixel signal; a selector that selects one of the plurality of comparison voltages according to a result of the comparison between the reference voltage and the pixel signal by the comparator; and a counter that counts a number of clocks until the selected comparison voltage and the pixel signal become equal while the comparator compares the comparison voltage selected by the selector and the pixel signal.