CMOS Photon-Counting Readout With Overflow-Bit Rolling Readout
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Solution Overview
Problem
Photon-counting imagers face challenges in transferring large amounts of digital data quickly enough, leading to high transfer bandwidth requirements, which can be prohibitively fast for large arrays and compromise spatial resolution and dynamic range.
Innovation Solution
Incorporating a digital counter with an overflow bit in each sensing element, coupled with a rolling readout system and frame store, allows for efficient data transfer by polling the overflow bit and storing data, reducing transfer bandwidth and power dissipation while maintaining high dynamic range and spatial resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a digital counter with many bits is incorporated in each sensing element to increase dynamic range, then the counter can count a large number of detected photons, but the sensing element size increases, limiting spatial resolution
Solution Approach 1:
The counter is divided into two parts: a small digital counter in each sensing element and a larger frame store located externally. The small counter handles local counting with limited bits, while the frame store provides extended storage capacity. This segmentation allows the sensing element to remain compact while achieving high dynamic range through the distributed frame store architecture.
2Measurement precision
If a digital counter with many bits is incorporated in each sensing element to increase dynamic range, then the counter can count a large number of detected photons, but the transfer bandwidth increases with the number of bits
Solution Approach 1:
The majority of the counter storage capacity is extracted from the sensing element and placed in the external frame store. Only a small portion of the counter (with fewer bits) remains in each sensing element. This extraction reduces the amount of data that must be transferred during each readout cycle, thereby reducing transfer bandwidth requirements while maintaining high dynamic range through the external frame store.
3Reliability
If each sensing element is interrogated once every ten nanoseconds to read out each photon, then data loss is avoided, but the readout rate becomes prohibitively fast for large arrays
Solution Approach 1:
The small digital counter in each sensing element continuously counts photons locally without requiring external interrogation for each detection event. This preliminary action allows the sensing element to accumulate multiple photon detections autonomously. The frame store is updated at a much lower rate by receiving data from multiple sensing elements simultaneously, thereby reducing the effective readout rate while preventing data loss through continuous local counting.
4Area of moving object
If the counter size is reduced to decrease sensing element area, then spatial resolution improves, but the dynamic range is limited by counter overflow
Solution Approach 1:
The small digital counter in each sensing element is nested within a larger frame store structure. The small counter handles local photon counting with limited capacity, and when it overflows, the data is transferred to the frame store which provides additional nesting levels of storage capacity. This nested architecture allows the compact sensing element to achieve high dynamic range through the hierarchical storage structure, maintaining both small size and high measurement precision.
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 results in lower transfer bandwidth, higher dynamic range, and smaller sensing elements, enabling better spatial resolution and lower power dissipation, making it suitable for high-performance imaging applications.
Implementation Method 1
avalanche photodiodes (APDs), which can be biased above breakdown to operate in Geiger mode. When an APD operating in Geiger mode detects a single photon, the APD generates a pulse
Data Source
AI summary
Embodiments of the present invention include complementary metal-oxide-semiconductor (CMOS) readout architectures for photon-counting arrays with a photon-counting detector, a digital counter, and an overflow bit in each of the sensing elements in the array. Typically, the photon-counting detector is a Geiger-mode avalanche photodiode (APD) that emits brief pulses every time it detects a photon. The pulse increments the digital counters, which, in turn, sets the overflow bit once it reaches a given count. A rolling readout system operably coupled to each sensing element polls the overflow bit, and, if the overflow bit is high, initiates a data transfer from the overflow bit to a frame store. Compared to other photo-counting imagers, photon-counting imagers with counters and overflow bits operate with decreased transfer bandwidth, high dynamic range, and fine spatial resolution.