Image Sensor Comparator Input Range Expansion via Replica Circuit
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Solution Overview
Problem
Image sensors face challenges in utilizing comparators due to limitations in their input range, which are affected by the distribution of process result values and temperature of semiconductor chips, leading to inadequate performance in varying light conditions.
Innovation Solution
The implementation of a replica circuit that transfers information to a ramp voltage generator to compensate and expand the input range of comparators, allowing for improved voltage comparison and signal generation across different light conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the input range of comparators is limited due to process result value distribution and temperature variations, then the device complexity is reduced, but the comparator utilization and performance under varying light conditions deteriorate
Solution Approach 1:
The replica circuit performs preliminary characterization of the comparator's input range by measuring the ramp voltage at which the output transitions during the integration period. This preliminary measurement is used to generate a control signal that adjusts the ramp voltage generator's output range, enabling the comparator to adapt to varying light conditions without requiring complex real-time adjustment mechanisms
Solution Approach 2:
The system implements feedback by using the replica circuit to measure the comparator's actual input range characteristics and feeding this information back to the ramp voltage generator. The control signal generated from the replica circuit's measurement results dynamically adjusts the ramp voltage range, creating a closed-loop system that optimizes comparator utilization based on process variations and temperature conditions
2Productivity
If the ramp voltage range is fixed, then the device complexity is reduced, but the comparator cannot be sufficiently utilized due to process result value distribution and temperature variations
Solution Approach 1:
The replica circuit creates a simplified copy of the comparator's critical path, including the integration capacitor and feedback transistor, to characterize the input range without requiring multiple full comparators. This copying approach enables accurate measurement of the ramp voltage transition point while maintaining low device complexity
Solution Approach 2:
The system dynamically changes the ramp voltage parameter by adjusting its output range based on control signals from the replica circuit. This parameter adjustment allows the ramp voltage to adapt to process result value distribution and temperature variations, maximizing comparator utilization across different operating conditions
Data Source
AI summary
An image sensor includes a pixel array including a plurality of pixels connected to row lines extending in a first direction and to column lines extending in a second direction intersecting the first direction, a ramp voltage generator configured to output a ramp voltage, a plurality of comparators, each of the plurality of comparators including a first input terminal to which the ramp voltage is input, and a second input terminal connected to one of the column lines, and a replica circuit having a same structure as a structure of a portion of the comparators. Each of the comparators includes a plurality of transistors, a first auto-zero transistor connected to the first input terminal, a second auto-zero transistor connected to the second input terminal, and wirings connected to the plurality of transistors, the first auto-zero transistor, and the second auto-zero transistor.