Image Sensor A/D Conversion Using Parallel Ramp Comparators
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Solution Overview
Problem
Existing imaging technologies face challenges in achieving high image quality due to trade-offs between gradation increase and conversion speed, leading to noise generation and deteriorated image quality, particularly when increasing the number of pixels or using multiple comparators.
Innovation Solution
An imaging element that includes comparing units comparing analog signals with ramp waves of different inclinations, counters that output digital data, and a sample-and-hold unit that holds signals before comparison, allowing for higher gradation without sacrificing frame rate by managing noise through electrical cutting of the vertical signal line and averaging count values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the number of pixels is increased to improve space resolution, then image quality is improved, but device complexity and power consumption increase
Solution Approach 1:
Multiple A/D converters are merged into a single integrated A/D conversion unit that processes signals from multiple pixels simultaneously. The unit includes multiple comparators that can compare analog signals from different pixels against reference voltages in parallel, reducing the overall number of separate conversion circuits needed while maintaining high space resolution through increased pixel count
2Measurement precision
If the number of output bits per pixel is increased to improve color reproducibility, then image quality is improved, but conversion speed decreases and frame rate is sacrificed
Solution Approach 1:
The A/D conversion process is segmented into multiple parallel comparison operations. Multiple comparators simultaneously compare the analog pixel signal against different reference voltages to determine multiple output bits in parallel, rather than sequentially. This segmentation of the conversion process into concurrent comparison stages enables high-bit-depth color reproduction while maintaining fast conversion speeds and frame rate
Solution Approach 2:
The reference voltage generation is made dynamic through a ramp waveform that continuously changes voltage levels. The ramp voltage dynamically sweeps through the signal range, allowing multiple comparators to capture different bit information simultaneously at different voltage thresholds, enabling high-precision color reproduction without sacrificing conversion speed
3Productivity
If multiple comparators are connected to one pixel vertical signal line to improve frame rate, then conversion speed is improved, but noise is generated due to kickback and image quality deteriorates
Solution Approach 1:
A sample-and-hold circuit is introduced as an intermediary between the pixel vertical signal line and the multiple comparators. This intermediary circuit captures and holds the analog pixel signal at a specific moment before comparison, isolating the signal source from the comparators during the comparison process. This prevents kickback noise from the comparators from coupling back into the pixel signal line, allowing multiple comparators to operate in parallel for high frame rate without degrading image quality
4Device complexity
If a single ramp wave reference voltage is used for A/D conversion, then device complexity is reduced, but gradation cannot be increased and manufacturing precision is limited
Solution Approach 1:
A single ramp wave reference voltage is made dynamic by continuously sweeping it through the full signal range. As the ramp voltage dynamically changes over time, multiple comparators with different threshold levels can simultaneously compare the pixel signal against different points on the ramp waveform, effectively creating multiple reference levels from a single dynamic reference source. This enables high-precision gradation without requiring multiple static reference voltage generators
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 enables the acquisition of higher image quality images by reducing noise and maintaining frame rate, while also decreasing power and area consumption compared to previous methods.
Implementation Method 1
a sample-and-hold unit that holds the analog signals supplied through the vertical signal line before the comparing units comparing the analog signals and the reference signals
Data Source
AI summary
The present disclosure relates to an imaging element, a control method, and an imaging apparatus that enable an image of a higher image quality to be obtained.An imaging apparatus according to the present disclosure includes a plurality of comparing units that compare analog signals supplied from pixels including photoelectric conversion elements through a vertical signal line with reference signals of ramp waves having different inclinations and output comparison results, a plurality of counters that execute counts until the comparison results are inverted for the different comparing units and output count values as digital data, and a sample-and-hold unit that electrically cuts the vertical signal line and the comparing units and supplies the held analog signals to the comparing units while the comparing units execute the comparisons. The present disclosure can be applied to an imaging element, a control method, and an imaging apparatus.