Column-Parallel Sigma-Delta Conversion Using Shared Reference Regulation
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Solution Overview
Problem
Sigma-delta modulation sensing circuits and analog-to-digital converters in CMOS imagers face noise issues due to the low capacitance of the floating diffusion region, affecting the accuracy of pixel and reset signal voltage conversions.
Innovation Solution
A sigma-delta modulation sensing circuit with a regulation branch using a reference voltage common across multiple columns, modulating resistance to generate an adjustment current that mitigates noise by subtracting the difference between reset and pixel signal voltage levels, and provides gain control for the imager.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sigma-delta modulation sensing circuits are used in CMOS imagers, then analog-to-digital conversion is achieved, but noise is excessive due to low capacitance of the floating diffusion region
Solution Approach 1:
The sensing circuit is divided into multiple independent column-parallel sigma-delta modulation circuits, each handling a specific column of pixels. This segmentation allows independent optimization of each column's noise performance while maintaining overall system efficiency.
Solution Approach 2:
A differential sensing architecture is introduced as an intermediary mechanism, where pixel signals are converted to differential voltages before further processing. This differential representation cancels common-mode noise and improves signal-to-noise ratio during the conversion process.
2Device complexity
If conventional sensing circuits are used, then circuit structure is simple, but noise mitigation and gain control are insufficient
Solution Approach 1:
The sensing circuit incorporates dynamically adjustable parameters including variable gain control and adaptive noise filtering. The circuit can adjust its operating characteristics in real-time to optimize performance under different lighting and noise conditions.
Solution Approach 2:
Feedback mechanisms are integrated into the sigma-delta modulation circuits, allowing the system to monitor and correct its own performance. This includes feedback for noise cancellation and gain stabilization, improving overall reliability without requiring overly complex external control systems.
3Object-affected harmful factors
If floating diffusion region capacitance is increased to reduce noise, then kT/C noise is reduced, but pixel cell area increases
Solution Approach 1:
Instead of increasing capacitance within the planar pixel cell area, the solution moves the capacitance function to a subsequent stage in the signal processing chain. The sigma-delta modulation circuits provide the necessary capacitance in the analog-to-digital conversion stage, decoupling noise performance from pixel cell area constraints.
Solution Approach 2:
The sigma-delta modulation circuit acts as an intermediary between the low-capacitance floating diffusion region and the final digital output. This intermediary provides the necessary capacitance and noise filtering without requiring the pixel cell itself to be larger.
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
Substantially reduces noise in the conversion process, enabling more accurate digital code generation and allowing for gain control to adjust pixel signal intensity.
Implementation Method 1
a photosensor, for example, a photogate, photoconductor or a photodiode within a substrate for accumulating photo-generated charge in the substrate
Implementation Method 2
A sigma-delta modulation sensing circuit with a regulation branch using a reference voltage common across multiple columns, modulating resistance to generate an adjustment current
Data Source
AI summary
A sigma-delta modulation sensing circuit and an analog-to-digital converter for an imager that do not rely on the ratio of the reset and pixel voltage levels being sensed. The sensing circuit includes a regulation branch based on a reference voltage common across multiple columns of the imager. The regulation branch has an adjustable resistance that is modulated during the sensing operation, which creates an adjustment current that is applied during the sensing operation to a current associated with one of the reset and pixel signals. The sensing circuit and analog-to-digital converter can generate a digital code based on the difference between the reset and pixel signal voltage levels, which substantially mitigates noise associated with the pixel and reset signal voltages. The reference voltage can also be used as a gain control for the imager as well.