可應用於人工視網膜系統之低功率高動態範圍影像感測與電流刺激晶片

A Low-Power High Dynamic Range Sense-and-Stimulus CMOS Image Sensor for Artificial Retina Applications

李其霖(Chih-Lin Lee)

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/博士(2014年)

摘要

視網膜炎與老年性黃斑病變為現代常見的眼睛疾病，其發生原因乃是因為視網膜上的感光細胞因為疾病或遺傳而導致受損或退化，造成病患視力下降、視角萎縮甚至失明。針對上述兩種眼疾患者，在本論文中，我們提出兩種利用脈衝調變技術之低功耗、高解析度影像感測與電流刺激人工視網膜晶片設計，用以取代病患受損的視網膜感光細胞，期許讓病患能恢復原有視力。
第一個提出的設計為一個4096畫素、0.8伏特操作的感測與刺激視網膜晶，每一個畫素主要分為前端用來感測光源的脈衝頻率調變互補式金氧半導體影像感測器以及一個用來刺激細胞的雙相電流刺激器所組成。有別於一般的3T或4T影像感測器，脈衝頻率調變影像感測器可低電壓操作，其可使用之動態範圍也不會因為操作電壓下降而有所限制。為了取代受損的視網膜細胞功能，本設計所出之雙相電流刺激器可將前端所接收的光訊號轉換成可用來刺激細胞的電流訊號。接著透過後製程的方法在晶片中的每個畫素中長出對應的刺激電極，模擬受損感光細胞之能力。此晶片並且提供了三種操作模式可供使用: (1) 測試模式:可將脈衝訊號、電壓訊號及雙相電流訊號依序讀出，將有利於量測晶片規格與特性；(2)可編輯模式: 可將光感測功能關閉，僅保留雙相電流刺激器之功能，並且可依據使用者需求，編輯輸出電流之二維圖形與大小，將有利於晶片測試階段量測使用；(3)植入模式: 用於晶片植入使用，可將輸入輸出腳位縮減至四個訊號，其所需偏壓與時脈都可從內部產生，使其體積最小化，並將手術所需的複雜度降至最低。
第二個設計為一個雙電壓操作且具有脈衝寬度調變技術之感測與刺激視網膜感測器。有別於第一個設計，本晶片採用脈衝寬度調變技術來增強感光電路的表現，此感光電路僅需0.5伏特操作並且具有固定圖形雜訊消除之技術。為了避免刺激電流因為操作電壓下降而隨之衰減，本設計將雙相電流感測器維持在1.8伏特電壓下操作，使其電流能維持足夠能力刺激視網膜細胞。利用本設計提出的時間電壓轉換器電路，可將感光電路之輸出與刺激電路之輸入進行連結並提供較好的抑制雜訊能力。除此之外，為了模仿人眼可自動適應強弱光環境變化的能力，自動動態範圍調變技術也在此設計電路中被提出。本晶片先利用外部電路將訊號進行平均並判斷環境光強度等級，接著針對系統增益進行調變，使輸出電流增益可因外在光強度自動進行調變。

關鍵字

低功率 ； 高動態範圍 ； 影像感測器 ； 人工視網膜

並列摘要

Retinitis pigmentosa (RP) and age-related macular degeneration (AMD) are diseases that notably affect photoreceptors of the retina and cause progressive vision loss. This thesis proposed two pulse modulation CMOS imager with integrated sense-and-stimulus (SAS) techniques, which are used to replace the sensing capability of damaged photoreceptor.
The first work exhibits a 0.8 V CMOS SAS imager with 4096 pixels for retinal prosthesis. The pixel consists of a pulse-frequency modulation (PFM) photon sensor (for sensing) and a balanced current-mode stimulator (for stimulating) to achieve a highly integrated and low-power solution for high-resolution vision recovery. With the PFM CMOS image sensor, an ultra-low-power operation is achieved. Three operation modes (test mode, programming (PG) mode, and implanted (IP) mode) have been implemented for various purposes. In test mode, the internal signals are multiplexed out serially for chip verification. In PG mode, the output pattern of current stimulator array is programmable by external addresses for patterned electrical stimulus experiments of retina. In IP mode, the chip is fully functional with a minimized number of I/O as 4 for in vivo operation.
The second work exhibits a dual-supply high dynamic range (HDR) SAS CMOS imager with adaptive gain control for artificial retina applications. When compared with the first work, the 0.5 V operated pulse-width modulation (PWM) based HDR image sensor is adopted to reduce power consumption with dynamic range extension. In this PWM sensor, the proposed threshold-variation cancelling (TVC) scheme is adopted to efficiently eliminate the FPN and achieve a low-noise image quality in the front-end. The 1.8 V operated in-pixel pulse-to-current stimulator provides a biphasic current pulse with sufficient intensity to activate neuron cells for artificial vision recovery applications. The time-to-voltage (T-V) conversion technique with a programmable gain is employed to achieve a reduced fixed-pattern-noise (FPN) and an adaptive sensitivity. The cellular mechanism in the retina varies its sensitivity at different intensity of light. An adaptive gain control system is also proposed to modulate a suitable response for mimicking the mechanism of retinal cells.

並列關鍵字

low power ； high dynamic range ； image sensor ； artifiticial retina

參考文獻

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