Pixel Output Rail Precharge for Faster Voltage Readout
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Solution Overview
Problem
CMOS image sensors face limitations in reading voltage levels from photodiodes efficiently, particularly in terms of settling time for reset and signal levels, due to the use of voltage follower transistors and current sources, which are slow and influenced by capacitance and current values.
Innovation Solution
A pixel array design with a variable impedance coupling and differential amplifier control, allowing for faster settling times by isolating or charging the voltage rail before reading, using switches to control voltage rails and transistors to manage impedance equivalent to an open circuit, and temperature-compensated reference voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a voltage follower transistor configuration is used to read the voltage level from the photodiode, then the reading operation can be performed, but the settling time for reset and signal levels becomes long due to capacitance and current source limitations
Solution Approach 1:
The patent applies preliminary action by charging the output voltage rail to the expected signal level before the actual reading operation. A switch connects the output voltage rail to a voltage source that pre-charges the rail to the anticipated voltage level, so when the reading operation begins, the rail is already at or near the correct voltage, significantly reducing the settling time required for the voltage follower transistor to stabilize.
Solution Approach 2:
The patent implements dynamics by using a switch that can dynamically change the impedance of the current source. During the pre-charging phase, the switch connects the output voltage rail to the voltage source through a low-impedance path for fast charging. During the reading phase, the switch transitions to a high-impedance state (equivalent to open circuit) to isolate the rail and maintain voltage stability, thus adapting the system behavior to different operational phases.
2Measurement precision
If a current source is used to draw current on the output line for reading the voltage, then the voltage level can be read, but the operation speed is limited by the current source characteristics
Solution Approach 1:
The patent applies periodic action by dividing the reading operation into distinct phases: a pre-charging phase where the output voltage rail is rapidly charged to the expected voltage level, followed by a reading phase where the voltage is measured. This periodic switching between charging and measuring modes enables faster overall operation compared to traditional continuous current sourcing methods.
Solution Approach 2:
The patent changes the impedance parameter of the current source dynamically using a switch. During pre-charging, the switch provides a low-impedance path for rapid voltage establishment. During the reading phase, the switch transitions to high impedance, effectively isolating the rail and enabling fast voltage measurement without the speed limitations of conventional current sources.
3Loss of time
If the variable impedance is controlled based on voltage on the second voltage rail, then faster settling times are achieved, but the device complexity increases with additional control circuits
Solution Approach 1:
The patent implements feedback by using a differential amplifier that continuously monitors the voltage on the output voltage rail and adjusts the control voltage for the variable impedance accordingly. The differential amplifier compares the actual rail voltage with a reference voltage and generates an error signal that drives the variable impedance control, automatically maintaining the rail voltage at the desired level and achieving fast settling without complex external control circuits.
Solution Approach 2:
The patent applies self-service by enabling the output voltage rail to charge itself to the correct voltage level through the controlled variable impedance. The system uses its own internal voltage rail and differential amplifier to automatically establish and maintain the correct voltage without requiring external pre-charging circuits or additional control mechanisms, thus reducing overall device complexity while achieving fast settling.
Data Source
AI summary
The present description concerns a pixel array comprising one or a plurality of pixels (PIX1). Each pixel comprises a first transistor having its control node coupled to a photodiode, a first main conduction node coupled to a first output voltage rail (VS), and a second main conduction node coupled to a second voltage rail (VCS). The array comprises a variable impedance (404) coupling the first voltage rail (VS) to a first power supply rail (VDD) and a current source (402) coupling the second voltage rail (VCS) to a second power supply rail (GND), the variable impedance (404) being controlled based on a voltage on the second voltage rail (VCS). The array comprises a first switch (4002) coupling the second voltage rail (VCS) to a third voltage rail (VINIT1).