Variable-Gain ADC for CMOS Image Sensor Dynamic Range
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Solution Overview
Problem
The increasing saturation charge amount and decreasing power voltage in CMOS image sensors make it difficult for the dynamic range of ADCs to cover pixel signals, leading to degraded dynamic range and noise performance in AD conversion, especially at dark parts of images with large tone differences.
Innovation Solution
A sensor and electronic device with an AD conversion unit that includes a comparator and an attenuation unit, where the electric signal is attenuated based on its amplitude during AD conversion, allowing the dynamic range to be expanded without degrading quantization noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a low analog gain is set to the ADC to increase the dynamic range, then the dynamic range is improved, but the quantization step width increases which degrades quantization noise at dark parts
Solution Approach 1:
The patent applies dynamics by making the analog gain variable rather than fixed. The gain is dynamically adjusted based on the input signal amplitude: high gain is applied to small signals (dark parts) to maintain precision, while low gain is applied to large signals to prevent saturation and maintain dynamic range. This dynamic adaptation resolves the contradiction between dynamic range and quantization noise.
Solution Approach 2:
The patent changes the parameter of analog gain from a static value to a variable parameter that is adjusted according to signal conditions. By changing the gain parameter adaptively based on input signal amplitude, the system achieves both wide dynamic range and low quantization noise across different signal levels.
2Reliability
If the saturation charge amount of pixels is increased to improve pixel characteristics, then the dynamic range coverage becomes more difficult due to decreased power voltage
Solution Approach 1:
The patent uses dynamic gain adjustment to adapt to the changed operating conditions. With increased saturation charge amount and decreased power voltage, the system dynamically selects appropriate gain levels to ensure the ADC can properly cover the pixel signal range while maintaining measurement precision.
Solution Approach 2:
The patent adjusts the analog gain parameter to compensate for the effects of increased saturation charge amount and decreased power voltage. By changing the gain parameter adaptively, the system maintains proper dynamic range coverage despite the changed pixel characteristics and power conditions.
Data Source
AI summary
Provided is an AD conversion unit that includes a comparator to compare an electric signal with a reference signal having a variable level, and performs AD conversion of the electric signal by using a result of the comparison between the electric signal and the reference signal by the comparator. An attenuation unit attenuates the electric signal supplied to the comparator with the amplitude of the electric signal.