Image Sensor Analog Sampling Circuit Noise Reduction
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Solution Overview
Problem
Image sensor devices in integrated circuits face random telegraph noise due to temporal fluctuations in the threshold voltage of source follower transistors, which is undesirable and currently cannot be effectively reduced without increasing transistor size or decreasing pixel fill factor.
Innovation Solution
An analog sampling circuit that samples and averages reset and video sampling values at multiple instants in time using multiple capacitors to produce average values, reducing random telegraph noise without increasing transistor size or reducing fill factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If the area of the source follower transistor is reduced to decrease pixel size, then the pixel area is reduced, but the temporal fluctuations in threshold voltage increase causing increased random telegraph noise
Solution Approach 1:
The sampling process is segmented into multiple temporal samples instead of a single sample. The analog sampling circuit takes multiple samples at different instants during the sampling phase, then averages them to reduce random telegraph noise. This segmentation of the sampling process allows small transistors to be used while maintaining signal quality.
Solution Approach 2:
The invention uses periodic sampling action by taking multiple samples at different time instants during the sampling phase. The sampling occurs periodically at different moments, and the results are averaged to reduce temporal fluctuations in threshold voltage that cause random telegraph noise.
2Reliability
If multiple samples are taken and averaged to reduce random telegraph noise, then noise is reduced, but the sampling time increases
Solution Approach 1:
The multiple sampling operations are performed continuously during the sampling phase without interruption. The analog sampling circuit continuously samples the pixel output at different instants throughout the sampling phase, maximizing the use of available time while reducing noise through averaging.
3Reliability
If the size of the source follower transistor is increased to reduce threshold voltage fluctuations, then random telegraph noise is reduced, but the pixel area increases and fill factor decreases
Solution Approach 1:
The invention replaces the mechanical/physical approach of increasing transistor size to reduce noise with an electronic signal processing approach. Instead of making the transistor physically larger to reduce threshold voltage fluctuations, the circuit uses multiple temporal samples and averaging to achieve the same noise reduction effect, thereby maintaining small transistor sizes and small pixel area.
Data Source
AI summary
An analog sampling circuit comprising a plurality of capacitors is used to sample the reset and video sampling levels at different instants in time to obtain a plurality of respective reset sampling values and a plurality of respective video sampling values. The reset sampling values are then averaged to obtain an average reset sampling value. Likewise, the video sampling values are averaged to obtain an average video sampling value. By averaging the reset sampling values and the video sampling values over time in this manner, random telegraph noise in the reset and video sampling values is eliminated or at least substantially reduced.