用於植入式生醫元件之低功率十位元每秒五十萬次取樣差量調變逐次漸進式類比數位轉換器

Abstract

在癲癇偵測與刺激的應用上，為了消除掉外部雜訊干擾，可植入性偵測元件需要滿足低功率要求。類比數位轉換器為其主要消耗功率電路之一。在眾多型態的類比數位轉換器裡，採用連續估算暫存器的方式為目前在焦耳對位元比表現上最為優越，但其無效率的切換方式，浪費了額外的功率與時間於不斷的找尋最終解。這裡提出一個對輸入訊號調變方式且改用預測其解的範圍，來降低額外切換電容所產生的功率消耗於冗餘的位元決定上。調變增加猜中率，藉由把前一個已解出來的數位碼，回授到輸入端並與目前取樣保持電路所取樣的類比值相減，來當作類比數位轉換器的輸入值。由於相鄰兩時間點，輸入訊號相似性頗高，故其相減值相當小。由成功大學蕭富勝教授領導團隊透過偵測實驗性老鼠的腦波。當其癲癇發作時，由外部裝置接收並放大，並以200Hz取樣率對其量化。放大後輸出電壓可達2V以上，但其相鄰兩點取樣值相減以500KHz做內差卻低於3mV。一般生理訊號範圍從0.5到2kHz，且多通道的設計可同時觀測大腦不同區域的變化，故設計八組通道對一類比數位轉換器以及取樣頻率為每秒五十萬次取樣。經由前置放大器將生理訊號放大，採用參考電壓為1.8V的類比數位轉換器只需十位元解析度。量測環境設定在輸入訊號頻率為100赫茲且參考電壓為1.8V下，量測結果為2.95μW和訊號對雜訊比為59.375dB，以及9.57的有效位元數。

For reducing outside noise in the applications of detection and stimulation of epilepsy, implantable epileptic devices require low power consumption. ADC is a power-hungry device in whole system. Delta-modulated SAR ADC using successive approximation register is power-efficiency than the other architectures, but conventional SAR ADC solving n-bit code form MSB to LSB is non-efficient. This work is proposed to modulate input signal and predicts the range to reduce redundant steps. Using the difference between the previously solved code and sampled input signal as input signal of ADC increases the hit rate of prediction, because in the same channel, the delta value between two successive samples is considerably small. Prof. F. Shaw and coworkers use external instruments to detect ECoG of Long-Evens rats. When epileptic seizure happened, the amplified voltage can reach above 2V. However, the delta value interpolated by 500kHz is less than 3mV. Since signal frequency of most of the bio-potential signals is in the range between 0.5Hz and 2KHz. Multi-channel design can simultaneously detect localized variations. Measured results of proposed delta-modulated SAR ADC show the total power consumption is 2.95μW, SNDR of 59.375dB, and ENOB of 9.57 at Fin=100Hz with supply voltage of 1.8V.