Capacitor-Switched Column Circuit for Early Differential Pixel Readout
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Solution Overview
Problem
Classic single-ended pixel signals lack the ability to reject noise, particularly from ground voltage differences and induction, and require conversion to fully differential signals for effective processing, which existing methods often necessitate active feedback circuits and additional circuitry.
Innovation Solution
A column circuit that converts single-ended or pseudo-differential pixel signals to fully differential signals using capacitors and switches, allowing for symmetrical differential signals on output nodes without the need for amplifiers, and can be integrated into existing image sensor circuitry, enabling early noise rejection and signal gain in the signal chain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended pixel signal is used, then the circuit is simple, but the signal cannot reject noise from ground voltage differences and induction
Solution Approach 1:
The single-ended signal is segmented into two complementary differential signals. The pixel signal is split across two capacitors (first and second capacitors) that are connected in opposite polarity, creating two output nodes that carry inverted versions of the same signal. This segmentation transforms a single vulnerable signal path into two complementary paths that can reject common-mode noise.
Solution Approach 2:
The patent employs asymmetric capacitor connections where the first capacitor connects the pixel signal to the first output node, while the second capacitor connects the pixel signal to the second output node in opposite polarity. This asymmetric arrangement around a virtual ground creates the differential signal structure needed for noise rejection while maintaining circuit simplicity.
2Object-affected harmful factors
If a pseudo-differential signal is used, then some noise may be rejected, but the signal is not fully differential and may not meet converter requirements
Solution Approach 1:
The patent inverts the traditional approach by connecting capacitors in opposite polarity rather than using a single capacitor with a reference voltage. The first capacitor connects the pixel signal to the first output node with one polarity, while the second capacitor connects the same pixel signal to the second output node with opposite polarity. This inversion creates truly differential outputs where both nodes carry active signal content rather than one node carrying a static reference voltage.
3Reliability
If a differential feedback amplifier is used for conversion, then fully differential signals are produced, but the circuit complexity increases
Solution Approach 1:
The patent extracts the essential differential signal generation function from the complex feedback amplifier circuit and implements it using simple passive capacitor-switch elements. By taking out the active amplification function and replacing it with passive capacitor-based signal splitting, the circuit achieves fully differential output without requiring complex active feedback circuits.
Solution Approach 2:
The patent replaces expensive, complex differential feedback amplifiers with inexpensive capacitor and switch elements. The capacitor-based differential generation circuit uses simple passive components that are cheaper and easier to integrate than active feedback amplifiers, while still achieving the required fully differential signal output.
4Device complexity
If conversion is performed late in the signal chain, then the circuit is simpler, but noise has more effect on the signal
Solution Approach 1:
The patent performs differential signal conversion at the earliest possible stage in the signal chain, immediately after pixel readout and before any further processing. By converting to fully differential signals early, the circuit establishes noise-rejection capability before signals pass through subsequent stages that could introduce additional noise or interference.
Data Source
AI summary
A column circuit for single-ended to fully differential conversion includes capacitors for sampling a pixel signal level, and capacitors for sampling a pixel reset level; and switches for connecting capacitors for sampling the pixel signal level and capacitors for sampling the pixel reset level in opposite polarity, thereby realizing symmetrical differential signals on two output nodes. A method is provided for single-ended to fully differential conversion, and a use of the column circuit for single-ended to fully differential conversion.