Active Reset Circuit for Single-Slope ADC Reset Spread Reduction
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Solution Overview
Problem
Modern image sensors face challenges in reducing reset signal variation, which limits their dynamic range and ability to operate effectively under high analog gain, as the variation in reset voltages overwhelms the input voltage range of ADCs, restricting the usable dynamic range and noise reduction capabilities.
Innovation Solution
The implementation of an active reset method using a single-slope ADC with a digital-to-analog converter that outputs a ramp waveform, allowing for P-phase and D-phase measurements with reduced noise variations by stabilizing the reset level through a differential comparator connected to the reset transistor, effectively minimizing reset signal variations and enhancing measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the gain of the amplifier is increased to improve signal amplification, then the signal amplification capability is improved, but the reset voltage variation is amplified causing the ADC input voltage range to be insufficient
Solution Approach 1:
The patent applies preliminary action by performing a reset measurement (P-phase) before the actual signal measurement (D-phase). The reset voltage is measured and stored in advance, then used to compensate for reset variations during the signal measurement phase. This allows the system to operate at high gain levels while maintaining accurate measurements by removing the harmful reset variation effect beforehand.
2Object-affected harmful factors
If correlated double sampling is used to reduce noise, then the noise reduction capability is improved, but the reset signal variation must be minimized which conflicts with high analog gain operation
Solution Approach 1:
The patent implements feedback by measuring the actual reset voltage in the P-phase and using this feedback information to compensate for reset variations in the D-phase measurement. The measured reset voltage is stored and then used to adjust the signal measurement, effectively feedback-correcting the reset variation effect and enabling both high gain operation and accurate noise-reduced measurements.
3Measurement precision
If the ADC input voltage range is reduced to accommodate reset variation, then the reset signal variation is controlled, but the dynamic range for photodiode signal is reduced
Solution Approach 1:
The patent extracts and removes the reset variation component from the total voltage range by measuring and compensating for it separately. The reset voltage is measured in the P-phase and then subtracted from or compensated in the D-phase measurement, effectively extracting the harmful variation component and allowing the full ADC range to be used for the photodiode signal, thereby preserving dynamic range while controlling reset variation.
Data Source
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AI summary
An image sensor comprises a pixel circuit including a reset transistor and configured to output a pixel signal; and a differential comparator including a pixel input, a reference input, and a comparator output, wherein one of a source or a drain of the reset transistor is connected to the comparator output. In this manner, an active reset method may be incorporated in the image sensor.