Image Sensor ADC Auto-Zeroing Using Previous Row Pixels
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Solution Overview
Problem
In image photographing devices with 3-T transistor schemes, the need for a separate auto-zeroing reference voltage complicates the design and increases area requirements, as the pixel cannot be flushed during readout time, necessitating additional row pixels for auto-zeroing operations.
Innovation Solution
An analog-to-digital converting circuit that utilizes a previous row pixel for which the readout operation is completed to generate a reference voltage for auto-zeroing, integrating a ramp signal generator and auto-zero controller to perform auto-zeroing operations, thereby eliminating the need for additional row pixels and reducing area requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a separate auto-zeroing reference voltage is applied to 3-T pixels, then auto-zeroing operation can be performed, but additional row pixels are required which increases area
Solution Approach 1:
The pixel array is designed to serve dual purposes: imaging pixels for capturing images and the same pixels serving as auto-zeroing reference sources when not actively being read out. This multi-functionality eliminates the need for separate dedicated reference pixels, thereby reducing overall area while maintaining reliable auto-zeroing operations.
Solution Approach 2:
Previously read-out pixels are recovered and reused as auto-zeroing reference sources. Instead of dedicating specific pixels exclusively for reference functions, the system discards the imaging function of already-read pixels temporarily and recovers them for auto-zeroing operations, optimizing area utilization.
2Reliability
If additional row pixels are added for auto-zeroing, then auto-zeroing reference voltage can be generated, but device complexity increases
Solution Approach 1:
The control logic is designed to manage pixels in a multi-functional manner, switching between imaging and auto-zeroing reference roles based on readout status. This universal approach simplifies the overall device structure by eliminating dedicated reference pixel rows while maintaining the necessary functionality through intelligent resource allocation.
Solution Approach 2:
The pixel array serves itself by using its own previously read-out pixels as auto-zeroing references. This self-service mechanism eliminates the need for external or dedicated reference sources, reducing device complexity while ensuring reliable auto-zeroing reference voltage generation.
3Reliability
If pixel readout time is extended to flush pixels, then auto-zeroing reference can be obtained, but readout speed decreases
Solution Approach 1:
Pixel flushing is performed in advance during the readout process itself, so that by the time auto-zeroing is needed, the pixels are already prepared and ready to serve as reference sources. This preliminary action during normal readout eliminates the need for extended waiting time, maintaining readout speed while ensuring pixel readiness for auto-zeroing.
Solution Approach 2:
The readout process continues uninterrupted while simultaneously preparing pixels for auto-zeroing functionality. Instead of pausing or extending readout time, the system maintains continuous useful action by integrating the flushing preparation into the normal readout flow, thereby preserving speed while ensuring reliability.
Data Source
AI summary
An analog-to-digital converting circuit includes: an analog-to-digital converter suitable for performing an analog-to-digital conversion on pixel signals of a plurality of pixels provided in a pixel array; a ramp signal generator suitable for providing a ramp signal to the analog-to-digital converter; and an auto-zero controller suitable for providing a reference voltage to the analog-to-digital converter to perform an auto-zeroing operation by using a row pixel for which a readout operation is performed by the analog-to-digital converter.