Image Sensor Smear Cancellation via Digital Offset Subtraction
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Solution Overview
Problem
Image sensors experience smear distortion due to bright objects affecting neighboring pixels during simultaneous readout, caused by coupling of shared control and reference signals, which is undesirable and difficult to mitigate.
Innovation Solution
An integrated-circuit image sensor with a smear cancellation circuit that determines an average digital code value and scales it to generate an offset value, which is then subtracted from individual pixel signals to eliminate or reduce smear.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multiple ADCs read out pixels simultaneously in a row-based pattern, then readout efficiency is improved, but smear distortion increases due to coupling from bright pixels to shared signals affecting dark pixels
Solution Approach 1:
The patent implements a feedback mechanism where the digital output codes from ADCs are fed back to a smear cancellation circuit. This circuit calculates the smear component based on the distribution of digital codes and subtracts it from the original signals. The feedback loop continuously monitors and corrects smear distortion, allowing simultaneous readout of multiple pixels while maintaining image quality by dynamically compensating for coupling effects between bright and dark pixels.
2Device complexity
If shared control signals and references are used among large sets of pixels and ADCs, then device complexity is reduced, but smear is generated through coupling between pixel readout chain and shared signals
Solution Approach 1:
The patent introduces an intermediary smear cancellation circuit that mediates between the pixel readout chain and the final image output. This intermediary component receives digital codes from ADCs, calculates the smear component caused by coupling to shared signals, and subtracts it before producing the final corrected pixel values. This approach allows the use of shared control signals and references (maintaining low device complexity) while eliminating the harmful smear effect through digital processing.
3Speed
If many pixels with large signals are read out simultaneously, then readout speed is improved, but the disturbance to shared signals increases proportionally, affecting especially dark pixels with small signals
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the smear cancellation factor based on the distribution of digital codes from simultaneously readout pixels. The smear cancellation circuit calculates the average digital code and uses it to determine the appropriate cancellation amount. This adaptive parameter adjustment allows fast simultaneous readout of many pixels while maintaining measurement precision by compensating for signal-dependent coupling effects that disproportionately affect dark pixels.
Data Source
AI summary
An apparatus of reading out an image sensor having a plurality of pixels arranged in rows and columns includes a plurality of analog-to-digital converters (ADCs) configured to output digital codes representative of received pixel signals of pixels coupled to respective ADCs, and a smear cancellation circuit configured to determine an average value of the digital codes, multiply the average value by a factor to obtain an offset value, and subtract the offset value from the digital codes to obtain smear-cancelled digital codes.