Single-Slope ADC Noise Reduction via Multi-Step Pixel Signal
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Solution Overview
Problem
CMOS image sensors face a trade-off between operating speed and power consumption, and existing single-slope ADCs generate noise due to coupling between adjacent columns, especially in high illumination conditions, which is exacerbated by shot noise.
Innovation Solution
A single-slope comparison device that generates multiple crossings by globally applying an offset to the pixel signal during analog-to-digital conversion, utilizing a multi-step pixel signal generation circuit, comparison circuit, and control circuit to detect crossings and adjust the ramp signal, thereby reducing noise and improving signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single-slope ADC uses a continuous time signal, then optimal performance is realized, but if the continuous time signal becomes a discrete time signal in the middle of the analog-to-digital conversion, unwanted noise is generated due to the coupling between adjacent columns
Solution Approach 1:
The pixel signal is segmented into multiple discrete levels (multi-step pixel signal) rather than processed as a continuous signal. The signal generation circuit divides the analog pixel signal into N discrete voltage levels, which eliminates the continuous-time coupling noise between adjacent columns while preserving the essential information for ADC conversion.
Solution Approach 2:
A multi-step pixel signal generation circuit is introduced as an intermediary between the pixel array and the comparison circuit. This intermediary converts the continuous-time pixel signal into a multi-level discrete signal, acting as a buffer that prevents direct coupling noise transmission while enabling accurate analog-to-digital conversion.
2Measurement precision
If multi-sampling techniques are used to improve resolution and reduce noise, then measurement precision is improved, but device complexity and power consumption increase
Solution Approach 1:
Multiple sampling operations are merged into a single analog-to-digital conversion process. By converting the pixel signal into a multi-step discrete signal before comparison, the system achieves multi-sampling effectiveness in one conversion cycle, reducing the need for multiple separate sampling circuits and operations.
Solution Approach 2:
The pixel signal parameter is changed from continuous-time analog to multi-level discrete levels. This parameter transformation enables the system to achieve high-resolution conversion with simpler circuitry, as the discrete multi-level signal can be processed by a single comparison circuit rather than requiring multiple sampling and averaging operations.
3Illumination intensity
If the incident amount of light increases, then signal strength is improved, but shot noise becomes dominant in a high illumination period
Solution Approach 1:
The pixel signal is pre-processed into a multi-step discrete signal before the ADC conversion process. This preliminary action of discretizing the signal at multiple levels reduces the impact of shot noise that occurs during high illumination conditions, as the noise is minimized before the critical comparison and conversion stages.
Data Source
AI summary
A comparison device includes a multi-step pixel signal generation circuit coupled to a pixel to receive a pixel signal and generate a multi-step pixel signal based on the pixel signal according to a multi-step trigger signal and a multi-step size control signal; a comparison circuit coupled to the multi-step pixel signal generation circuit to compare a ramp signal with the multi-step pixel signal and output a comparison signal; and a control circuit coupled to the multi-step pixel signal generation circuit and the comparison circuit to detect crossings of the multi-step pixel signal and the ramp signal based on the comparison signal from the comparison circuit and output the multi-step size control signal to the multi-step pixel signal generation circuit.