Image Sensor Bus Driver Feedback Impedance Control
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Solution Overview
Problem
Conventional CMOS image sensors face limitations in bus driving speed due to high switch impedance, which restricts readout speed and requires multiple buses, increasing manufacturing costs and complexity.
Innovation Solution
A pixel bus driver with dual feedback paths between a column amplifier and the pixel bus, including a transistor M1 as a switch and transistors M2 and M3, which adjusts impedance and settling time through external bias voltage control, enhancing driving speed and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a common bus is used to connect different columns via switches, then the image sensor can be manufactured with simpler structure, but the bus driving speed is limited due to high switch impedance
Solution Approach 1:
A buffer stage is introduced as an intermediary component between the column switch and the common bus. This buffer stage acts as a mediator that isolates the high-impedance switch from the bus, allowing the switch to maintain its simplicity while the buffer provides low-impedance driving capability to the bus, thus resolving the contradiction between manufacturing simplicity and bus driving speed.
Solution Approach 2:
The patent implements a feedback mechanism where the output of the column amplifier is fed back through the switch to the input of the amplifier. This feedback path allows the system to maintain signal integrity and improve the effective driving speed by compensating for the high switch impedance through the regenerative action of the amplifier feedback loop.
2Speed
If multiple buses are used to increase readout speed, then the bus driving speed per bus can be maintained, but the device complexity and die area increase
Solution Approach 1:
The buffer stage serves as an intermediary that enables a single bus to achieve higher effective readout speed by compensating for the switch impedance. This eliminates the need for multiple buses, thereby reducing device complexity and die area while maintaining or improving readout speed.
Solution Approach 2:
The patent changes the impedance parameter of the bus driving path by introducing the buffer stage. This parameter change allows the system to achieve higher readout speed with fewer buses by transforming the high-impedance switch output into a low-impedance bus drive capability.
3Speed
If the switch size is increased to reduce switch impedance, then the bus driving speed improves, but the total bus capacitance increases
Solution Approach 1:
The buffer stage is introduced as an intermediary between the switch and the bus, which decouples the relationship between switch size and bus capacitance. The buffer provides current amplification capability, allowing a small switch to drive the bus effectively without increasing bus capacitance, thus resolving the contradiction between driving speed and capacitance.
Solution Approach 2:
The patent replaces the direct electrical connection (mechanical/electrical system) between the switch and bus with an active buffer stage that uses feedback control. This substitution allows the system to achieve low effective impedance without physically increasing switch size, thereby avoiding increased bus capacitance.
Data Source
AI summary
A bus driver having one or more feedback paths between a column amplifier and a pixel bus in an image sensor is described.