Snapshot Infrared Sensor Pixel Readout Circuit for High-Speed Imaging
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Solution Overview
Problem
Current uncooled infrared sensors face limitations in capturing infrared images quickly while maintaining a Noise Equivalent Temperature Difference (NETD) of 50 mK or less, as they require scanning rows or columns and have integration times that cannot be decreased without degrading performance.
Innovation Solution
Integrating a readout circuit within each pixel, including a transistor amplifier, feedback capacitor, and a skimming current mirror with a thermalized microbolometer offset outside the pixel, allowing for simultaneous reading of all pixels and reducing bias current to minimize thermal runaway and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If sequential scanning of rows or columns is used for reading pixels, then device complexity is reduced, but acquisition speed deteriorates
Solution Approach 1:
The readout circuit is segmented and integrated into each pixel independently. Each pixel contains its own transistor amplifier, feedback capacitor, and reset switch, allowing simultaneous reading of all pixels without sequential scanning, thereby achieving high acquisition speed while managing complexity through modular integration
Solution Approach 2:
The readout architecture transitions from a column-parallel sequential scanning approach to a fully parallel pixel-level integration. By moving the readout function to the pixel dimension rather than the column dimension, all pixels can be read simultaneously, achieving frame rates up to 500 fps
2Productivity
If integration time is decreased to improve acquisition speed, then productivity improves, but measurement precision deteriorates
Solution Approach 1:
A feedback capacitor is integrated at each pixel to store and accumulate the infrared signal during the integration period. This feedback mechanism allows sufficient signal accumulation even with short integration times (e.g., 2 ms), maintaining measurement precision (NETD ≤ 50 mK) while enabling high acquisition speeds (up to 500 frames per second)
Solution Approach 2:
The reset switch performs preliminary action by clearing the feedback capacitor before each new integration cycle. This ensures that each frame starts with a clean state, allowing precise measurement accumulation during the short integration period without interference from previous frames, thus maintaining NETD performance at high frame rates
3Measurement precision
If bias current is increased to improve signal strength, then measurement precision improves, but power consumption increases and thermal runaway occurs
Solution Approach 1:
The readout circuit replaces mechanical/thermal signal amplification methods with electronic amplification using transistor amplifiers integrated at each pixel. This electronic substitution allows for low bias current operation (reducing power consumption and thermal effects) while maintaining strong signal output through active electronic gain, achieving NETD ≤ 50 mK without excessive power consumption or thermal runaway
4Measurement precision
If common mode current is eliminated using compensation elements, then measurement precision improves, but device complexity increases
Solution Approach 1:
The common mode current compensation function is extracted from the pixel array and implemented separately using dedicated compensation structures and circuits. This separation allows for effective common mode rejection (improving measurement precision) while isolating the compensation complexity from the active pixel elements, managing overall device complexity through functional separation
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 enables rapid acquisition of infrared images up to 500 frames per second with a NETD of 50 mK or less, improving integration time and reducing power consumption while maintaining high sensitivity.
Implementation Method 1
each pixel integrating an imaging microbolometer... resistance variations of the imaging microbolometer, due to infrared radiations
Implementation Method 2
a function of absorption of the incident radiation, a function of conversion of the power of the absorbed radiation into thermal power
Implementation Method 3
a capacitor assembled in feedback on said transistor between said output node and said integration node... said capacitor integrates the difference between said current flowing through said imaging microbolometer and said skimming current
Implementation Method 4
a skimming current crossing said skimming control transistor being controlled according to the temperature of at least one thermalized microbolometer
Data Source
AI summary
An infrared sensor includes an assembly of pixels juxtaposed in rows and in columns, each pixel integrating an imaging microbolometer and an integrator assembly. The integrator assembly includes a transistor assembled as an amplifier, and a capacitor assembled in feedback on the transistor between an output node and an integration node. The integration node is connected to a skimming transistor operating as a current mirror with a skimming control transistor offset outside of the pixel. A skimming current flowing through the skimming control transistor is controlled according to the temperature of at least one thermalized microbolometer. The current mirror assembly enables to transmit the skimming current flowing through said skimming control transistor onto the integration node so that the capacitor integrates the difference between a current flowing through the imaging microbolometer and the skimming current.