Column Amplifier Capacitor Switch Circuit for CMOS Gain Control
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Solution Overview
Problem
Current image sensors face limitations in achieving optimal gain adjustment and power efficiency in their readout circuits, particularly in CMOS imaging systems, which affects the dynamic range and frame rate of captured images.
Innovation Solution
The implementation of a capacitor switch circuit within the column amplifier, allowing for adjustable gain settings by interleaving switches among capacitors, reduces parasitic capacitance and enhances bandwidth, thereby improving gain control and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If gain adjustment is implemented in conventional CMOS image sensors, then dynamic range is improved, but power consumption increases and frame rate decreases
Solution Approach 1:
The capacitor bank is segmented into multiple individually controllable capacitors (first capacitor, second capacitor, third capacitor, fourth capacitor) with different capacitance values. Each capacitor can be independently switched to contribute to the total feedback capacitance, enabling precise gain adjustment in discrete steps while maintaining efficient operation at each level.
Solution Approach 2:
The feedback capacitance is made dynamically adjustable through switch circuits that connect different capacitors to the summing node based on desired gain settings. This dynamic reconfiguration allows the system to adapt gain levels in real-time without requiring continuous power consumption, as switches consume minimal power when in steady state.
2Measurement precision
If gain adjustment is implemented in conventional CMOS image sensors, then dynamic range is improved, but frame rate decreases
Solution Approach 1:
The capacitor bank is segmented into multiple individually controllable capacitors (first capacitor, second capacitor, third capacitor, fourth capacitor) with different capacitance values. Each capacitor can be independently switched to contribute to the total feedback capacitance, enabling precise gain adjustment in discrete steps while maintaining efficient operation at each level.
Solution Approach 2:
The feedback capacitance is made dynamically adjustable through switch circuits that connect different capacitors to the summing node based on desired gain settings. This dynamic reconfiguration allows the system to adapt gain levels in real-time without requiring continuous power consumption, as switches consume minimal power when in steady state.
3Device complexity
If conventional capacitor configurations are used, then circuit simplicity is maintained, but parasitic capacitance increases and bandwidth decreases
Solution Approach 1:
The capacitor bank is segmented into multiple individually controllable capacitors (first capacitor, second capacitor, third capacitor, fourth capacitor) with different capacitance values. Each capacitor can be independently switched to contribute to the total feedback capacitance, enabling precise gain adjustment in discrete steps while maintaining efficient operation at each level.
Solution Approach 2:
Different capacitors are assigned different capacitance values (first capacitor has first capacitance value, second capacitor has second capacitance value, etc.) to optimize performance at different gain settings. This local differentiation allows the system to minimize parasitic effects and maximize bandwidth at each operating point by selecting the appropriate capacitor combination.
Data Source
AI summary
A pixel cell readout circuit includes an amplifier and a capacitor switch circuit that includes a first routing path coupled to an input of the amplifier. A second routing path includes switches coupled in series along the second routing path. A first end of the second routing path is coupled to a bitline. A second end of the second routing path is coupled to an output of the amplifier. Only one of the switches is turned off and a remainder of the switches are turned on. Capacitors are coupled in parallel between the first routing path and the second routing path. A first end of each of the capacitors is coupled to the first routing path. A second end of each of the capacitors is coupled to the second routing path. The switches are interleaved among the second ends of the capacitors along the second routing path.