Digital Camera Black Level Calibration Circuit
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Solution Overview
Problem
Existing digital camera image sensor readout techniques face challenges in achieving high stability for black level calibration due to variations in supply voltages and temperatures, which affect image quality.
Innovation Solution
A digital camera system with an image sensor and readout circuit that includes a compensator, programmable gain amplifier, analog-to-digital converter, digital black level calibration unit, and feed-forward control design, which generates black level calibrated digital data by subtracting a black reference from sensed signals and using a voltage ladder to create reference voltages adjusted according to sensor characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional black level calibration is used, then image sensor readout is simple, but black level stability deteriorates due to supply voltage and temperature variations
Solution Approach 1:
The readout circuit is segmented into multiple functional modules: compensator, programmable gain amplifier, analog-to-digital converter, digital black level calibration unit, and feed-forward control design. Each module handles specific tasks in the signal processing chain, allowing independent optimization and maintenance while maintaining overall system stability.
Solution Approach 2:
The feed-forward control design generates black reference signals in advance using a voltage ladder and gain stages, before the actual image capture. This preliminary generation of calibration references allows the system to compensate for supply voltage and temperature variations proactively, rather than reactively, maintaining black level stability without complex real-time adjustments.
2Manufacturing precision
If black level calibration is performed without compensation, then readout process is simple, but image quality deteriorates due to temporal offsets from environmental changes
Solution Approach 1:
The compensator unit provides feedback by continuously monitoring and adjusting the black reference signal based on detected variations in supply voltage and temperature. This feedback mechanism ensures that temporal offsets caused by environmental changes are compensated, maintaining high image quality through adaptive calibration.
Solution Approach 2:
The system dynamically changes calibration parameters including the black reference voltage levels and gain settings based on detected environmental conditions. The voltage ladder generates different reference voltage levels, and the programmable gain amplifier adjusts its gain setting, allowing the system to adapt to varying operating conditions and maintain consistent image quality.
3Adaptability or versatility
If fixed gain amplification is used, then circuit design is simple, but adaptability to different sensor characteristics deteriorates
Solution Approach 1:
The programmable gain amplifier provides dynamic adjustment of gain settings based on the specific sensor characteristics being processed. The amplifier can switch between different gain states to optimize performance for different sensor types and operating conditions, enhancing adaptability without requiring multiple fixed amplifier circuits.
Solution Approach 2:
The readout circuit is designed with universal components that can handle multiple sensor characteristics. The voltage ladder, multiplexer, and programmable gain amplifier work together to provide a single versatile calibration system that can adapt to different sensor types and conditions, eliminating the need for separate dedicated circuits for each sensor characteristic.
Data Source
AI summary
A digital camera and an image sensor readout method with black level calibration are disclosed. In a readout circuit of the digital camera, a black reference for black level calibration and an analog-to-digital conversion reference voltage for analog-to-digital conversion are generated from reference voltages provided by one voltage ladder. Device-to-device variations of sensor characteristics are considered when selecting the reference voltage for generating the black reference, and, a gain between the selected reference voltage and the black reference varies with a gain of a programmable gain amplifier coupled after a compensator. The compensator subtracts the black reference from a sensed signal that the readout circuit receives from an image sensor of the digital camera.