CMOS Image Sensor Readout With 1.5 ADC Conversions Per Pixel
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Solution Overview
Problem
Current CMOS image sensors require two analog to digital conversions per pixel for Digital Correlated Double Sampling (DCDS), leading to increased energy consumption and noise in image data.
Innovation Solution
A method that reduces the number of analog to digital conversions to 1.5 per pixel by directly generating the DCDS result from the ADC output, eliminating the need for arithmetic subtraction between signal and reset conversions, using a two-pixel-column wide SAR ADC with differential inputs and charge redistribution DACs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two analog to digital conversions are performed per pixel for DCDS, then noise reduction and signal quality are improved, but energy consumption increases
Solution Approach 1:
The patent merges the reset conversion and signal conversion operations by using the same ADC for both conversions and reusing the reset conversion result in the DCDS calculation. This combining approach maintains the noise reduction benefits of DCDS while reducing the total number of conversions and associated energy consumption.
Solution Approach 2:
The patent recovers and reuses the reset conversion result in the DCDS calculation formula (C = S - R + offset). Instead of discarding the reset conversion data after initial use, it is recovered and applied again in the final correlated double sampling computation, thereby maintaining signal quality without requiring additional conversions.
2Measurement precision
If two analog to digital conversions are performed per pixel, then DCDS noise reduction is achieved, but the number of conversions and processing complexity increase
Solution Approach 1:
The patent combines the ADC operations for reset and signal conversions, using a single ADC unit to perform both functions. This merging reduces the overall processing complexity while maintaining the dual-conversion DCDS noise reduction capability through efficient resource sharing and result reuse.
3Use of energy by moving object
If 1.5 analog to digital conversions per pixel are used, then energy consumption is reduced, but signal processing complexity increases
Solution Approach 1:
The patent recovers the reset conversion result and applies it in the DCDS formula (C = S - R + offset). This recovery approach enables the reduced 1.5-conversion-per-pixel scheme to achieve the same noise reduction performance as full DCDS, offsetting the increased processing complexity through intelligent data reuse and mathematical compensation.
Solution Approach 2:
The patent introduces an offset term as an intermediary element in the DCDS calculation to compensate for the reduced conversion operations. This offset mediator enables the system to maintain accurate noise reduction performance even with fewer conversions, balancing energy savings against processing complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces energy consumption per pixel and minimizes noise in image data while maintaining image quality, achieving efficient pixel readout with reduced signal processing.
Implementation Method 1
a two-pixel-column wide SAR ADC with differential inputs and charge redistribution DACs
Implementation Method 2
a two-pixel-column wide SAR ADC with differential inputs and charge redistribution DACs
Data Source
AI summary
A CMOS image sensor comprises an array of pixels. A column of the pixel array is coupled to a readout column. The readout column is couple to a readout circuitry (RC) that reads out image data from the pixel array. The RC comprises two sampling switches which are coupled to a 2-column successive approximation register (SAR) analog-to-digital converter (ADC). The 2-column SAR ADC comprises a differential comparator, a local SAR control, and two digital-to-analog converters (DACs). The two sampling switches are coupled between two readout columns and two inputs of the differential comparator, respectively. An image readout method reads two pixels with three conversions through the RC. The RC is operated by the local SAR control to set the two DACs based on comparator output, and upon which a reset digital value is obtained and stored. An overall reduced algorithm calculation is achieved herein.