Active Pixel Sensor Readout Feedback for Faster Column Settling
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Solution Overview
Problem
Conventional active pixel sensor imaging systems face challenges with high power consumption and settling time due to parasitic capacitances in readout column lines, particularly in large imagers, where increasing biasing current to enhance readout speed increases power consumption and decreases voltage headroom.
Innovation Solution
An active pixel sensor imaging system with a current sensing circuit and feedback circuit configured as a classAB current mirror, providing a closed loop around the pixel configuration to improve settling time without increasing power consumption, featuring a current mirror for sensing and feedback to manage dynamic current and slew-rate limitations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the biasing current of the column line is increased to increase readout speed, then the settling time is reduced, but the power consumption increases and the voltage headroom decreases
Solution Approach 1:
The patent employs a feedback circuit that senses the current through the readout transistor and provides feedback current to charge the column line parasitic capacitance. This feedback mechanism allows the system to achieve fast settling by dynamically compensating for the parasitic capacitance without requiring continuously high biasing current, thus resolving the contradiction between readout speed and power consumption
Solution Approach 2:
The patent transitions from a static biasing current approach to a dynamic current control system. The biasing current is made variable and adaptive, being increased only when needed for fast settling and reduced during normal operation. This dynamic adjustment allows the system to achieve high readout speed temporarily without sustaining high power consumption
2Speed
If the biasing current of the column line is increased to increase readout speed, then the settling time is reduced, but the voltage headroom decreases
Solution Approach 1:
The feedback circuit dynamically compensates for voltage drops across the column line by sensing the actual voltage conditions and adjusting the feedback current accordingly. This allows the system to maintain adequate voltage headroom even during high-speed readout operations, as the feedback mechanism compensates for voltage drops rather than relying on continuously high biasing current
3Use of energy by moving object
If a simple source-follower configuration is used, then the power consumption is reduced, but the settling time increases
Solution Approach 1:
The feedback circuit is designed to activate only when needed for fast settling, rather than operating continuously. During normal low-power operation, the simple source-follower configuration is maintained with minimal power consumption. When fast settling is required, the feedback current is activated to charge the parasitic capacitance, achieving fast settling without continuously sacrificing power efficiency
Solution Approach 2:
The feedback current is applied periodically or on-demand rather than continuously. The system switches between low-power mode (simple source-follower) and high-speed mode (with feedback current activation) based on the readout requirements, achieving both power efficiency and fast settling capability at different times
Data Source
AI summary
An active pixel sensor imaging system is disclosed. In one aspect, the system includes a plurality of active pixel sensor circuits arranged into an array of rows and columns. Each active pixel sensor is connected to a supply line and a column line, and operable to generate a voltage output through the column line corresponding to a detected light intensity. The system includes a current sensing circuit, located external to the plurality of active pixel sensor circuits and connected to the supply line. The current sensing circuit is implemented as a current mirror for sensing a current through an active pixel sensor circuit readout transistor. The system includes a feedback circuit, located external to the plurality of active pixel sensor circuits and connected to the column line, to a current generator and to the current sensing circuit. The feedback circuit is implemented as a classAB current mirror configured for controlled quiescent current.