Variable Gain Amplifier for Imaging Sensor Dynamic Range
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Solution Overview
Problem
Current imaging arrays face challenges in achieving high dynamic range while maintaining low cost and short readout time, particularly in low light conditions, due to increased silicon area and cost requirements from multiple transistors needed for improved dynamic range.
Innovation Solution
A novel bit line processing circuit using a capacitive transimpedance amplifier with variable gain and a fixed-step size ADC, allowing selectable digitization noise levels and gain adjustment based on signal strength, to optimize signal processing for both low and high light conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple readout amplifiers with different gains are used on each signal line to improve dynamic range, then the dynamic range of the readout circuit is increased, but the number of transistors in the processing circuitry is substantially increased, leading to increased silicon area and cost
Solution Approach 1:
The patent combines multiple amplification functions into a single variable gain amplifier. Instead of using multiple separate amplifiers with fixed gains, the invention implements one amplifier that can dynamically adjust its gain through capacitive feedback switching, thereby reducing the number of transistors while maintaining the ability to handle multiple signal levels
Solution Approach 2:
The patent employs a dynamic gain control mechanism where the amplifier's feedback capacitance is switched between different values based on the signal level. This dynamic adjustment allows the same amplifier to provide different effective gains for different signal conditions, replacing the need for multiple static amplifiers
2Measurement precision
If multiple readout amplifiers are used to increase dynamic range, then the readout circuit can handle both low and high signal levels, but the silicon area required to implement the readout array is significantly increased
Solution Approach 1:
The variable gain amplifier serves multiple functions: it amplifies low-level signals when high gain is selected and handles high-level signals when low gain is selected. This single multi-functional component replaces what would traditionally require multiple specialized amplifiers, thereby reducing the overall silicon area
Solution Approach 2:
The patent changes the amplifier's operating parameters (feedback capacitance value) based on signal conditions. By switching between different capacitance values, the amplifier's gain is adjusted to match the signal level, allowing one amplifier to perform the work of multiple amplifiers with different fixed gains
3Productivity
If the readout time is reduced to minimize noise in low light images, then the readout speed is improved, but the dynamic range requirements become more challenging to meet with a single amplifier
Solution Approach 1:
The rapid gain switching capability of the variable gain amplifier allows it to adapt to different signal levels during the fast readout process. The amplifier can quickly transition between gain states to accommodate the varying signal levels from different pixels, maintaining both speed and dynamic range performance
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 reduces digitization noise and maintains low cost by using a single capacitive transimpedance amplifier with variable gain, effectively increasing dynamic range without significantly increasing silicon area or cost, while ensuring accurate signal processing across varying light levels.
Implementation Method 1
The signal digitizing circuit includes a variable gain amplifier having a plurality of gain values that amplifies the voltage signal to generate an amplified signal that is digitized by an ADC having a fixed number of bits. In one aspect of the invention, the variable gain amplifier includes a capacitive transimpedance amplifier having a feedback capacitance that is determined by the output control signal.
Data Source
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AI summary
An imaging sensor using a novel bit line processing circuit, that circuit, and the method of processing the pixel outputs from an image sensor using that processing circuit are disclosed. The image sensor includes an array of pixel sensors, a signal digitizing circuit, and a digitizing controller. Each pixel sensor generates a voltage signal that is a function of a charge on the photodetector in that pixel sensor, and couples that voltage signal to a bit line in response to a first signal. The signal digitizing circuit is connected to the bit line, the digitizing circuit converting the voltage signal to a plurality of output digital values, the output digital values having selectable levels of digitization noise. The digitizing controller generates the level of noise based on the voltage signal. The signal digitizing circuit includes a variable gain amplifier and an ADC having a fixed number of bits.