低電壓與低功耗脈衝寬度調變影像感測器之研究

The Research of Low Voltage and Low Power Pulse-Width Modulation CMOS Imagers

邱衍智(Albert Yen-Chih Chiou)

指導教授 ： 謝志成

國立清華大學/電機資訊學院/電機工程學系所/博士(2018年)

摘要

在2015年的時代中，物聯網（Internet of Things）及其中的無線感測器網路（WSN）與人工智慧（artificial intelligent）及其中的機械視覺（machine vision）皆非常需要低功率CMOS的影像感測器來達成各式各樣的應用，而為了讓影像感測電路應用於不同的環境下，具備多種操作模式可以提供更好的適應性。為了達到低電壓操作，脈衝寬度調變（PWM）影像感測器具有低電壓的操作模式進而具備較低功耗的淺力。但此類PWM影像感測器容易受到晶片製作中製程的變異以及電路的雜訊影響，而使得其影像品質下降。這些原因激發了對PWM畫素在低電壓操作下的興趣，並且在0.18μm CMOS標準製程中進行研究以及驗證。本論文的內容架構會先講述整個研究的動機與目的，接著是傳統型的影像感測器，研究主題將會分成（a）高動態範圍（HDR）PWM影像感測器設計和（b）線性反應（LR）PWM影像感測器設計來進行研究，最後是本論文結論與未來發展。以下概述兩個設計的內容。
第一個設計內發展出了一個雙模式操作並可自供電的高動態範圍（HDR） PWM 影像感測器，包含HDR成像模式和能量收集（EH）模式。在HDR模式下，此設計提出了雙曝光延長計數（DEEC）的操作方式，並配合可程式化電流控制閥（PCCT）的產生器，實現了高達137dB的動態範圍。此晶片採用0.4V操作並於16.5fps的操作速度下消耗32.1μW，並達成9.8 fJ/pixel•code的iFoM。在EH模式下，影像感測器將畫素轉換為太陽能收集模式，並在60klux（白天）時產生455mV和14μW的能量來達到4.1fps的自供電操作。
第二個設計著重於雜訊的分析，並發展出了高影像畫質的LR模式。此設計晶片操作於0.4V下並具備300×200的PWM像素陣列。在LR模式中，發展出了自適應多次取樣（AMS）並配合雙方向ramp（DSR），來對弱光的畫素進行n次的取樣來降低雜訊並提升整個晶片的動態範圍和信噪比（SNR）。此晶片操作於0.4V並達成了60.1dB的動態範圍，+ 0.36 /-0.29％的非線性度，0.159％的固定圖案雜訊。HDR模式和EH模式也保留在這項工作中，以提供監控和自供電能力。

關鍵字

超低電壓 ； 高動態範圍 ； 線性反應 ； 能量收集 ； 脈衝寬度調變 ； 影像感測器

並列摘要

Low-power image sensors are highly demanded for Internet of Things (IoT), wireless sensor network (WSN), and machine vision of AI. In these applications, multiple imaging modes provide a good flexibility for different environment. Also, pulse-width modulation (PWM) imagers, as compared with traditional image sensor, have the advantages of low-voltage operation and potentially lower power. However, the image quality of PWM imagers are easily degraded by process variation and circuit noise that causes either spatial or temporal noise issues. These reasons motivate the study of PWM pixel for multiple imaging modes in standard 0.18μm CMOS technology. This thesis introduces the motivation at first, and then provides traditional voltage- and time-domain image sensors in second and third part, respectively. The fourth and fifth parts are the main content that demonstrates two major works: (a) a high-dynamic-range (HDR) PWM imager and (b) a linear-response (LR) PWM imager, both include energy harvesting operations. The conclusion and future work will be presented at the end.
The first work presents a self-powered HDR CMOS imager with dual-mode operation: HDR imaging mode and energy harvesting (EH) mode. In HDR mode, a dual-exposure extended-counting (DEEC) scheme is proposed with a programmable current-controlled threshold (PCCT) generator, which achieves a high DR of 137dB and a low iFoM of 8.1fJ/pixel·code. In energy harvesting mode, the sensing pixel turns out to be an energy harvesting pixels with an additional global micro solar cell, which generates 455mV and 14μW at 60klux (sunny day) and supports a self-powered imaging operation at 4.1fps.
The second work presents a PWM imager with three operation modes which includes an additional high quality LR mode. In LR mode, an adaptive-multiple-sampling (AMS) scheme is developed with dual-slope ramping (DSR), pixel-wise signal level judgment, and n-time multiple sampling to improve the signal-to-noise ratio (SNR) for low illuminance pixels. The reset noise, photon shot noise, and thermal noise of the PWM pixel are considered and analyzed to provide good image quality. The 0.4V linear-response PWM pixel achieves a LR DR of 60.1dB, a non-linearity of +0.36/-0.29%, and a FPN of 0.159%. HDR and EH mode are also preserved to provide monitoring and self-powered capability.

並列關鍵字

ultra-low-voltage ； high dynamic range ； linear response ； energy harvesting ； pulse-width modulation (PWM) ； CMOS image sensor

參考文獻

Bibliography

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