應用於心電圖之低頻交換式電容六階帶通濾波器

Sixth-order switched-capacitance band-pass filter for ECG application

劉麗珍(Li-Chen Liu)

指導教授 ： 龔正 ； 黃智方

國立清華大學/電機資訊學院/電子工程研究所/碩士(2009年)

摘要

本論文是設計應用於心電圖(electrocardiogram)的交換式電容帶通濾波器。從人體擷取到的心電訊號，必須先經由儀器放大器放大，再經過帶通濾波器將雜訊濾除，只擷取需要的頻寬範圍，其中帶通濾波器是由高通濾波器和低通濾波器組成。正常人的心臟跳動大約一分鐘60~100次，設計帶通濾波器的中心頻率為2.2Hz，其高通濾波器的3dB極點為0.05Hz，目的在濾除電路裡的直流偏移和直流漂移等低頻雜訊，避免造成後級電路飽和。再串接多級低通濾波器，有效並快速的濾除高頻雜訊，其3dB極點為100Hz，低頻增益為十倍，放大訊號以便觀察。
　　又因為濾波器所需要的頻段非常低頻，電路設計中的電阻電容值勢必非常大，且電阻的製程誤差大，採用交換式電容的技術取代原有的電阻，使原本需要幾十MEGΩ到幾百MEGΩ的電阻，只需要幾pF的電容即可達成，不僅降低面積的需求，且濾波器的轉換函數變成電容的比例，layout佈局更可以達到縮小誤差。
最後，採用電容放大器(Capacitance Multiplier)的技術。利用運算放大器的虛短路和相同尺寸相同阻抗的反相器等特性，組成比起通過原尺寸的電容還有大數倍電流的電路方塊，應用這種概念組成的電流放大器(Current Multiplier Cell)，串聯數個電路方塊，就可以將電容放大到很高的倍數，本論文設計了放大一萬倍的電容。

關鍵字

交換式電容 ； 濾波器 ； 電容乘法器

並列摘要

In this thesis a switched-capacitor band-pass filter was designed for the application of electrocardiogram (ECG). The ECG signal had been acquired from the body. First of all, the signal was gained by using instrument amplifier. Then, noise and unwanted signal was filtered out by using band-pass filter, which was consisted of high-pass filter series with low-pass filters. High-pass filter, which cutoff frequency was at 0.05Hz and filtered out the dc bias and dc offset to avoid circuit working in saturation region. Then, two stages of low-pass filters filtered out noise at high frequency effectively, which cutoff frequency were at 100Hz and had a gain of nine for observing easily.
Due to the ultra low frequency specific, large resistances and capacitances were needed. Therefore the technique of switched-capacitor equivalent to resistance was designed. This design of switched-capacitor made the system only need a few pF of the capacitance could achieve the specification in place of large resistances, not only saved the areas, but also transferred the function to a ratio of capacitances, and it also made less error of production.
Due to the large capacitances were designed, the technique of capacitance multiplier was adopted too, capacitance multiplier was consisted of series many stages of current multiplier cells (CMC). CMC was composed of operational amplifiers which input pairs were virtual ground and inverters which used the same size for the same impendence, the current would be multiplied and the capacitance would also be multiplied. In the thesis ten thousands of capacitances was designed.

並列關鍵字

switched-capacitor ； filter ； capacitance multiplier

參考文獻

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