Image Sensor Comparator Clamp for Accurate Conversion Timing
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Solution Overview
Problem
In solid-state image sensors, the shared power supply for the comparator leads to variations in drain voltage when the pixel signal coincides with the reference signal, causing timing errors in comparison results, resulting in non-linearity and degraded image quality.
Innovation Solution
The implementation of a comparator configuration with an output transistor receiving a drain-source voltage and an input-side clamp transistor to stabilize the drain voltage, ensuring accurate timing of comparison results.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the comparator shares the power supply with the pixel circuit to reduce power consumption, then power consumption is reduced, but the drain voltage varies when the pixel signal coincides with the reference signal, causing timing errors in comparison results
Solution Approach 1:
The power supply system is segmented into two independent parts: a first power supply for the pixel circuit and a second power supply for the comparator. This segmentation prevents the comparator from affecting the pixel circuit's power supply, eliminating drain voltage variations while maintaining low power consumption by keeping the comparator's power supply separate and optimized.
Solution Approach 2:
A clamp transistor is introduced as an intermediary element between the pixel signal line and the comparator's power supply. This clamp transistor stabilizes the drain voltage by controlling the power supply connection, preventing voltage variations that would cause timing errors while allowing the comparator to share the power supply structure.
2Device complexity
If the drain voltage varies according to pixel signal level, then the comparator can operate with shared power supply, but timing when the comparison result is inverted shifts from ideal timing, causing errors in digital signal
Solution Approach 1:
The clamp transistor performs preliminary stabilization of the drain voltage before the comparison operation occurs. By pre-establishing a stable voltage level through the clamp transistor, the comparator operates with consistent timing characteristics, preventing inversion timing shifts and ensuring accurate digital signal generation.
Solution Approach 2:
The power supply voltage parameter for the comparator is changed from a shared variable parameter (coupled to pixel signal variations) to a controlled parameter through the clamp transistor. This parameter change stabilizes the operating point of the comparator, ensuring consistent comparison timing and accurate digital output.
Data Source
AI summary
A solid-state image sensor includes: an input transistor configured to output, from a drain, a drain voltage according to an input voltage input to a source in a case where the input voltage substantially coincides with a predetermined reference voltage input to a gate; and an output transistor configured to output a signal indicating whether or not a difference between the input voltage input to a source and the drain voltage input to a gate exceeds a predetermined threshold voltage as a comparison result between the input voltage and the reference voltage.