Active Pixel Sensor Readout With Dynamic Bias Feedback
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Solution Overview
Problem
Conventional active pixel sensor imaging systems face challenges in reducing power consumption and settling time, particularly the downwards settling time, due to parasitic capacitances and voltage drop issues in large imagers, which are exacerbated by high-speed image sensor readout requirements.
Innovation Solution
The implementation of a closed-loop circuit with a current sensing circuit and a classAB feedback circuit, utilizing a current mirror configuration to dynamically adjust the tail current of the source follower, allowing for different current gains for rising and falling slopes of the column line output voltage, thereby improving settling time without increasing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the biasing current of the column line is increased to increase readout speed, then the readout speed is improved, but the power consumption increases and the voltage headroom decreases
Solution Approach 1:
The patent applies dynamics by making the biasing current dynamic rather than static. A feedback circuit continuously monitors the column line voltage and adjusts the biasing current in real-time based on the actual operating conditions. This allows the system to use higher current only when needed (during transitions) and lower current during stable periods, resolving the contradiction between speed and power consumption.
Solution Approach 2:
The patent implements feedback by introducing a voltage sensing circuit that monitors the column line voltage and feeds this information back to a control circuit. The control circuit then adjusts the biasing current accordingly. This closed-loop feedback mechanism enables the system to automatically optimize the trade-off between readout speed and power consumption based on actual signal conditions.
2Speed
If the biasing current of the column line is increased to increase readout speed, then the readout speed is improved, but the voltage headroom decreases
Solution Approach 1:
The patent makes the biasing current dynamic by using a feedback circuit that adjusts the current in real-time based on the column line voltage level. During signal transitions, the current is temporarily increased to maintain fast settling, while during stable periods, the current is reduced to maintain adequate voltage headroom. This dynamic adjustment resolves the contradiction between speed and voltage headroom.
Solution Approach 2:
The voltage sensing feedback circuit monitors the column line voltage and provides feedback to the current control circuit. When the voltage drops below a threshold, the feedback triggers an increase in biasing current to restore the voltage quickly, thereby maintaining voltage headroom while enabling fast readout when necessary.
3Loss of time
If a current sensing circuit with feedback is implemented to increase readout speed, then the settling time is reduced, but the implementation complexity increases
Solution Approach 1:
The patent implements a feedback circuit that senses the column line voltage and adjusts the biasing current accordingly. This feedback mechanism reduces settling time by providing additional current during voltage transitions. The complexity is managed by using a relatively simple voltage divider-based sensing circuit and a basic operational amplifier-based control circuit, rather than more complex current sensing approaches.
Solution Approach 2:
The patent introduces an intermediary voltage sensing circuit that indirectly monitors the column line conditions without requiring direct current sensing. This voltage-based intermediary approach simplifies the implementation compared to direct current sensing methods, as voltage can be monitored more easily and with less circuit complexity than current.
4Loss of time
If a higher biasing current is used to reduce settling time, then the settling time is reduced, but the power consumption increases
Solution Approach 1:
The patent applies dynamics by making the biasing current variable rather than constant. The feedback circuit detects when settling is needed (during signal transitions) and temporarily increases the current to reduce settling time. Once settling is complete, the current is reduced to minimize power consumption. This dynamic current adjustment resolves the contradiction between settling time and power consumption.
Solution Approach 2:
The feedback-controlled biasing current operates in a periodic manner, switching between high current (during transitions requiring fast settling) and low current (during stable periods). This periodic action pattern allows the system to achieve fast settling when needed while maintaining low average power consumption, resolving the contradiction between settling time and power usage.
Data Source
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AI summary
An active pixel sensor imaging system (100) comprising: a plurality of active pixel sensor circuits (P) arranged into an array of rows and columns, each active pixel sensor being connected to a supply line (SL) and a column line (CL) and operable to generate a voltage output (VCL) through the column line (CL) corresponding to a detected light intensity; a current sensing circuit (CS1), located external to the plurality of active pixel sensor circuits (P) and connected to the supply line (SL), the current sensing circuit being implemented as current mirror for sensing a current through an active pixel sensor circuit readout transistor (M1); and a feedback circuit (FC1), located external to the plurality of active pixel sensor circuits (P) and connected to the column line (CL), to a current generator (IS) and to the current sensing circuit (CS1), the feedback circuit being implemented as a classAB current mirror configured for controlled quiescent current. And a device comprising an active pixel sensor imaging system according to the present description.