Comparator Offset Cancellation for Event Sensor Threshold Symmetry
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Solution Overview
Problem
Conventional pixel circuits in event-based vision sensors suffer from comparator offset variations due to internal component mismatches, leading to asymmetric and inconsistent detection thresholds across pixels, which degrade sensitivity to illumination changes.
Innovation Solution
A comparator circuit with a control circuit that operates in normal and calibration modes, using external calibration voltages to set symmetric and stable detection thresholds by canceling out internal offsets through reset and calibration switches, and a unified control signal for event read-out.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional comparator circuits are used in event-based sensors, then the device complexity is low, but the measurement precision of detection thresholds deteriorates due to comparator offsets
Solution Approach 1:
The patent applies preliminary action by performing offset calibration before normal operation. The calibration phase pre-determines the offset values by temporarily applying calibration voltages and measuring the comparator outputs, then storing these offset values for use during subsequent normal detection operations. This separates the offset measurement from the event detection process, improving threshold precision without significantly increasing operational complexity.
Solution Approach 2:
The patent introduces intermediary calibration voltages and control switches as mediators between the comparator inputs and the detection process. These calibration voltages are applied to one input of the comparator during calibration mode, allowing the offset to be measured and compensated. The control switches act as intermediaries to selectively connect calibration or normal operation voltages, enabling offset cancellation without permanently altering the comparator structure.
2Reliability
If comparator offset cancellation is implemented, then the reliability of event detection improves, but the device complexity increases due to additional calibration circuitry
Solution Approach 1:
The patent merges the calibration functionality with the existing comparator circuit by integrating calibration switches and control logic into the same pixel circuit structure. The calibration switches are combined with the reset switches, and the control circuit that manages calibration mode is integrated with the existing event detection control logic. This merging approach improves detection reliability through offset cancellation while minimizing the increase in overall device complexity by reusing existing circuit elements.
Solution Approach 2:
The control circuit is designed with multi-functionality, serving both normal event detection operations and calibration operations. The same control circuit generates signals for both the reset switches and the calibration switches, and manages both event readout and offset calibration sequences. This universal control approach improves reliability through consistent offset compensation while avoiding the need for separate dedicated calibration control circuitry.
3Measurement precision
If calibration mode is added to the comparator circuit, then the consistency of detection thresholds across pixels improves, but the productivity of event processing decreases due to calibration time
Solution Approach 1:
The patent implements periodic calibration action where the calibration mode is activated at specific intervals rather than continuously. The calibration can be performed periodically across rows or columns of pixels, with each pixel undergoing calibration at predetermined times. This periodic approach ensures threshold consistency across all pixels while minimizing the impact on overall event processing productivity by confining calibration to discrete time windows.
Solution Approach 2:
The patent performs calibration as a preliminary action before normal event processing begins, or during idle periods when no events are being processed. By completing the calibration phase in advance, the system establishes consistent thresholds for all pixels before event detection starts, ensuring measurement precision without interrupting ongoing event processing. This preliminary calibration approach separates the setup phase from the operational phase, maintaining high productivity during event detection.
Data Source
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AI summary
A comparator circuit for detecting an amount of variation in an input signal (Vlog) is provided, comprising a comparator having a non-inverting input (Vref_pos, Vref_neg) and an inverting input (Vcp, Vcn); a capacitor (Cp, Cn) connected between the inverting input and a terminal for applying the input signal as a voltage (Vlog); a reset switch (SWrp, SWrn) connected between the inverting input and an output (EPOS, ENEG) of the comparator; and a calibration switch (SWcp, SWcn) connected to select for the non-inverting input a voltage among a threshold voltage (Vth_pos, Vth_neg) and a calibration voltage (Vcal_pos, Vcal_neg).