Infrared Detector Read-Out Cells With In-Cell Offset Current Subtraction
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Solution Overview
Problem
The sensitivity and dynamic range of bolometer arrays, particularly in thermal imaging, are limited by offset currents that are not effectively removed, leading to noise addition and fabrication complexities when attempting to subtract these currents using additional resistors.
Innovation Solution
In-cell current subtraction is achieved through the use of a voltage ramp signal generated by a reference pixel and a feedback amplifier, which is applied to read-out cells to subtract current prior to integration, with optional temperature-dependent and temperature-independent current sources to adjust for temperature variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a resistor is added in series with each bolometer resistor to remove offset current, then the offset current can be removed, but noise is added and pixel unit cell space is insufficient
Solution Approach 1:
The patent merges the current subtraction function with existing circuit elements by applying a voltage ramp signal to the integration capacitor. This approach combines multiple functions (signal integration and offset current subtraction) into a single circuit node, eliminating the need for additional series resistors and maintaining pixel unit cell space constraints while achieving offset current removal.
Solution Approach 2:
The patent changes the temporal parameter of the voltage applied to the integration capacitor by introducing a time-varying ramp signal. This ramp signal's voltage increases linearly over time, creating a current that precisely matches and subtracts the offset current component, thereby improving sensitivity without adding physical resistors to the pixel structure.
2Measurement precision
If a resistor is added to subtract offset current, then the offset current can be removed, but fabrication process is complicated due to different process requirements
Solution Approach 1:
The patent merges the current subtraction function with existing circuit elements by applying a voltage ramp signal to the integration capacitor. This approach combines multiple functions (signal integration and offset current subtraction) into a single circuit node, eliminating the need for additional series resistors and maintaining pixel unit cell space constraints while achieving offset current removal.
Solution Approach 2:
The patent changes the temporal parameter of the voltage applied to the integration capacitor by introducing a time-varying ramp signal. This ramp signal's voltage increases linearly over time, creating a current that precisely matches and subtracts the offset current component, thereby improving sensitivity without adding physical resistors to the pixel structure.
3Device complexity
If offset current is not removed, then the circuit is simple, but the voltage range available for signal is limited
Solution Approach 1:
The patent changes the temporal parameter of the voltage applied to the integration capacitor by introducing a time-varying ramp signal. This ramp signal's voltage increases linearly over time, creating a current that precisely matches and subtracts the offset current component, thereby improving sensitivity without adding physical resistors to the pixel structure.
Solution Approach 2:
The system uses a feedback amplifier to generate the voltage ramp signal based on the output from a reference pixel. This feedback mechanism automatically adjusts the ramp signal to match the actual offset current level, ensuring accurate subtraction while maintaining circuit simplicity and maximizing the voltage range available for signal detection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly improves the sensitivity of read-out cells by effectively removing offset currents, reducing noise, and expanding the voltage range available for signal detection, while simplifying the fabrication process and maintaining sensitivity across varying temperatures.
Implementation Method 1
a feedback amplifier, which is applied to read-out cells to subtract current prior to integration
Data Source
AI summary
Read-out cell systems are disclosed for image detectors, including infrared image detectors, that provide improved sensitivity by providing in-cell subtraction through the use of a voltage ramp signal generated using a reference pixel and a feedback amplifier. The ramp voltage is generated using a reference pixel and an amplifier having feedback. The ramp voltage is then provided to a plurality of read-out cells. The ramp voltage can be coupled to an input transistor to provide current subtraction prior to the integration node. The ramp voltage can also be provided to integration capacitors within the read-out cells to provide current subtraction directly to the integration node. Further, a temperature-independent fixed current source can also be utilized to further control current subtraction.