Column Bias Switching in Image Sensors for Faster Low-Noise Readout
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Solution Overview
Problem
Image sensors face challenges in reducing noise components and settling time during readout operations, which affect their operating speed and noise properties.
Innovation Solution
The image sensor employs a bias circuit with multiple current circuits and a switching circuit that maintains transistors in a turn-on state during readout, reducing noise components and settling time by stabilizing bias currents across column lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If transistors are turned off during readout operation to save power, then power consumption is reduced, but noise components increase and settling time lengthens
Solution Approach 1:
The bias circuit is segmented into multiple independent current circuits, each serving specific column lines. This allows selective control of bias current supply to different regions, enabling power saving in non-active areas while maintaining low noise in active readout regions.
Solution Approach 2:
The switching circuit dynamically controls the connection between column lines and current circuits based on readout phases. During reset phase, transistors are kept on to reduce noise; during pixel reading phase, transistors can be turned off to save power, optimizing both noise performance and power consumption at different times.
2Use of energy by moving object
If transistors are turned off during readout operation, then power consumption is reduced, but settling time increases
Solution Approach 1:
The bias circuit pre-charges column lines and maintains transistors in on-state before actual pixel reading begins. This preliminary action ensures that when reading starts, the circuit is already in a low-impedance state, minimizing settling time while allowing power saving during the actual readout phase.
3Object-generated harmful factors
If separate current circuits are used for each column line to minimize noise, then noise components are reduced, but device complexity increases
Solution Approach 1:
Each current circuit is designed to serve multiple column lines through the switching circuit. A single current circuit can be dynamically connected to different column lines at different times, providing universal functionality that reduces the total number of current circuits needed while maintaining low noise performance.
Solution Approach 2:
Multiple column lines are merged to share common current circuits through the switching circuit. This consolidation reduces the overall circuit complexity and component count while maintaining noise performance by ensuring that shared circuits are properly isolated and controlled during simultaneous operations.
4Object-generated harmful factors
If more current circuits are used to reduce noise, then noise components are reduced, but the area occupied by the bias circuit increases
Solution Approach 1:
Current circuits are designed with multi-functional capability to serve multiple column lines through time-multiplexed switching. This universality allows fewer current circuits to achieve the same noise reduction effect that would otherwise require more circuits, thereby reducing the total area occupied by the bias circuit.
Data Source
AI summary
An image sensor includes a first column line and a second column line configured to extend in a first direction, a plurality of pixel groups configured to connect to the first column line or the second column line and to comprise a plurality of pixels in each of the plurality of pixel groups, a bias circuit configured to comprise a first current circuit and a second current circuit configured to output different bias currents in a first operational mode, and a switching circuit configured to connect the first column line to the first current circuit and connect the second column line to the second current circuit during a first time period, and to connect the first column line to the second current circuit and connect the second column line to the first current circuit during a second time period subsequent to the first time period in the first operational mode.