摘 要： 便攜式動態(tài)心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" 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title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號" title="心電信號">心電信號采集器" title="數(shù)據(jù)采集器">數(shù)據(jù)采集器是現(xiàn)代動態(tài)心電圖分析系統(tǒng)的重要組成設(shè)備。介紹以單片機為核心的智能心電數(shù)據(jù)采集系統(tǒng)的原理，詳細(xì)論述了硬件設(shè)計的要點及數(shù)據(jù)預(yù)處理軟件的有關(guān)算法。
關(guān)鍵詞： 單片機 心電信號 數(shù)據(jù)采集
心電圖是心臟疾病診斷的重要工具之一，然而在臨床中一些心臟病患者在短時間內(nèi)其心電圖并無異常。這樣就要花相當(dāng)長的時間在醫(yī)院中作觀察診斷，從而給病人和醫(yī)院都造成了不必要的麻煩。為此，動態(tài)心電圖分析處理系統(tǒng)便應(yīng)運而生，該系統(tǒng)可以分為兩大部分，一是攜帶在被檢查者身上的袖珍心電圖信號采集器，另一為由微機系統(tǒng)組成的心電圖處理診斷系統(tǒng)。被檢查者配帶了心電圖數(shù)據(jù)采集器后可離開醫(yī)院，其一天的動態(tài)心電信號由采集器記錄下來，然后交院方將記錄的數(shù)據(jù)回送微機心電圖處理系統(tǒng)中進行分析，作出正確的診斷。目前此類系統(tǒng)大多為歐美產(chǎn)品，價格昂貴，國內(nèi)的研究尚處在起步階段。由于所需存儲的信息量大，目前的典型產(chǎn)品大多采用高精度的盒式磁帶機作為數(shù)據(jù)記錄盒。從原理上來說，不可避免地存在磁頭的機械磨損問題。同時，磁頭的清潔度、機械振動及走帶的穩(wěn)定性都會影響心電信號記錄的準(zhǔn)確性。另外，將磁帶上記錄的數(shù)據(jù)回送微機系統(tǒng)時，還需專用設(shè)備，其回送的速度相對也較慢。隨著單片機技術(shù)的發(fā)展，特別是超大容量存儲器的出現(xiàn)，我們采用以單片機為核心，擴充大容量數(shù)據(jù)存儲器的方式，徹底彌補了由磁帶式數(shù)據(jù)記錄器造成的上述不足。
1 心電數(shù)據(jù)采集器的硬件設(shè)計
1.1 基本硬件結(jié)構(gòu)
心電數(shù)據(jù)采集器的整個電路主要包括心電信號放大電路、模數(shù)轉(zhuǎn)換器、外接實時時鐘、單片機、程序存儲器、數(shù)據(jù)存儲器、與IBM PC的并行通訊接口等。其基本原結(jié)構(gòu)如圖1所示。
采集器采集的信號為雙通道心電信號。大容量數(shù)據(jù)存儲器擴展采用端口擴展譯碼。關(guān)于輸入過載保護、采樣、A/D轉(zhuǎn)換、與 IBM PC并行通訊、外接實時時鐘等問題已有相當(dāng)多的文獻作了介紹，本文不再詳述。下面主要就心電信號的專用放大電路和心電數(shù)據(jù)采集系統(tǒng)的動態(tài)校零技術(shù)進行論述。
1.2 輸入信號放大電路
該心電數(shù)據(jù)采集器的二組差動輸入信號是通過電極取自人體表皮的緩變微弱信號，其值不超過5mV。我們采用二級放大器構(gòu)成放大電路，其結(jié)構(gòu)如圖2所示。
第一級由A1、A2和A3組成同相并聯(lián)差動運算放大電路。為了取得最佳的低頻響應(yīng)，必須將該級的輸入端直接耦合到電極上，因此我們將該級的增益設(shè)計得較低，以減小該級的輸入偏流，避免使電極極化產(chǎn)生高壓。A1、A2組成同相差動帶增益的電壓跟隨器，大大提高了電路的輸入阻抗，它對共模信號有很高的抑制比，對差模信號的放大倍數(shù)為：
A3為一獨立的差動式放大器，調(diào)節(jié)Rf可使電阻對稱，減小誤差，進一步提高共模抑制比。通過計算可得第一級的綜合放大倍數(shù)為:
第二級由A4形成同相比例放大電路，其放大倍數(shù)為32。
二級放大總的增益為800。由于心電信號的頻率范圍要求放大器具有0.04～150Hz的帶寬，所以利用第二級放大器A4，通過C1和C2組成了有源帶通濾波電路，以抑制干擾。
放大電路中另一個值得注意的問題是當(dāng)異聯(lián)電極換接時，由于C1的作用可能會引起放大器的輸出飽和。因此在電路中加入電子開關(guān)S0，當(dāng)放大器輸出飽和時，通過比較器自動驅(qū)動S0瞬時接通，C1迅速充電，從而使輸出脫離飽和狀態(tài)。
在該放大電路的設(shè)計中，雖然可使用常規(guī)低偏流運放，但從實際出發(fā)，我們優(yōu)先選用單電源運放，如F3140。由于其輸入級差分電路采用了PMOS器件，使它的輸入電阻可高達1.5兆歐，具有極低的輸入失調(diào)電流、電壓，并且可省去調(diào)零電路和外接偏置電阻，使整個放大電路變得更為緊湊，調(diào)試更為方便，也降低了功耗。
1.3 心電數(shù)據(jù)采集器的動態(tài)校零
輸入級電路雖然采用了具有補償及自動調(diào)零的運放電路，但并不能完全克服放大器的零點漂移。當(dāng)設(shè)備連續(xù)工作時其影響尤為明顯。因此，在心電數(shù)據(jù)采集的硬件設(shè)計時，我們還采用了動態(tài)校零技術(shù)，取得了良好的效果。使用該技術(shù)，在硬件電路上只需作簡單的改動，即用兩個同步工作的模擬電子開關(guān)，如AH5012連接在輸入端與放大器之間，如圖3所示。
以A通道為例，每次正式采樣前，先由CPU發(fā)出校零控制信號，使S1斷開，S2接通。接著讀入“零”位信號得V0, 并將其存入零位值寄存單元。然后復(fù)位校零控制信號，即使S1接通，S2斷開，采樣讀入迭加了“零”位信號的心電信號數(shù)據(jù)VA0,顯然實際的信號采樣值即為：
VA＝VAo－V0 (4)
對于模擬電子開關(guān)導(dǎo)通電阻引起的“零”位讀數(shù)偏差，由于它是一個穩(wěn)定的常量，可以通過實驗分析，由軟件編程進一步予以消除。
2 系統(tǒng)應(yīng)用軟件設(shè)計
2.1 軟件總體結(jié)構(gòu)
整個心電數(shù)據(jù)采集器的軟件采用模塊化結(jié)構(gòu)，以便于維護和擴充。該軟件主要包括初始化、動態(tài)校零、采樣、并行通訊管理、WDT等常規(guī)模塊，還包括心電信號數(shù)據(jù)預(yù)處理的專用濾波軟件模塊，如50Hz交流干擾濾波及漂移濾波等。系統(tǒng)程序流程圖如圖4所示。
2.2 交流干擾濾波算法
考慮到采集器攜帶者可能接觸的環(huán)境因素，在采集到的心電信號數(shù)據(jù)中必然會存在一定的50Hz工頻干擾信號。鑒于心電信號的頻譜主要在0.5～100Hz范圍內(nèi)，因此不能采用常規(guī)的平均濾波或點阻濾波算法。為此，我們選用了適用于心電信號數(shù)據(jù)的“增量預(yù)估”濾波算法。此算法主要原理是通過三角恒等式和一些采樣數(shù)據(jù)，估算出正弦波出現(xiàn)的幅值，經(jīng)過計算判斷該交流信號是否存在于采樣信號中，如有則減去，使實際心電圖信號保存下來。顯然，正確預(yù)估50Hz正弦信號的幅值是整個算法的關(guān)鍵。其主要依據(jù)是利用三角恒等式：
SIN(θ+δ)＝2SINθCOSδ－SIN(θ－δ) (5)
式中：θ＝2πfkt；δ＝2πft；
f＝50Hz；k＝0,1，.....
可以認(rèn)為δ表示工頻正弦干擾信號在一個采樣周期內(nèi)的相位變化。由式5可從兩次采樣(相位為θ 和θ－δ)的干擾幅值預(yù)估出下一次采樣時( 相位為(θ+δ)的正弦波干擾信號幅值。將式5乘以振幅可得50Hz
工頻干擾的差分方程：
DK+1＝2DKCOS(2πft)－DK－1 (6)
式中DK為交流干擾幅值序列，T為采樣周期。
為了在遞推算法中不斷修正預(yù)估值與實際干擾值的偏差，我們作誤差估計函數(shù)
F＝(AK－DK)－(AK－1－DK－1) (7)
式中，AK為可能包含交流干擾的實際采樣值。由于在兩次采樣間隔內(nèi)，AK 與AK－1是十分接近的，從而由式7可知，在算法的初始階段如果DK估計過大，則F趨于較大的負(fù)值；反之F趨于較大的正值。當(dāng)F接近于零時，說明交流干擾信號的預(yù)估值接近實際值。因此在濾波程序中，通過判斷F的大小就可確定在DK上加上或減去其修正值Δd。當(dāng)然要適當(dāng)選擇Δd，如果Δd 取得太小會影響算法的收斂速度，而太大會造成振蕩。初始干擾幅度我們選為0.2mV。
2.3 漂移濾波算法
在心電信號數(shù)據(jù)采集過程中，由于許多因素(如電極或皮膚的張力)都可能引起緩慢的偏置漂移。目前根據(jù)歐美的標(biāo)準(zhǔn)，要求心電圖輸入設(shè)備至少有3.2秒的時間常數(shù)，這樣會進一步加重漂移的影響。為此我們仿照克服漂移的經(jīng)典的模擬濾波器，經(jīng)離散化后得到了漂移濾波算式，實用后取得了滿意的結(jié)果。模擬濾波器的傳遞函數(shù)為：
式中，τ＝1.024s
對式8求Z變換得
對應(yīng)的差分方程為：
由式9不難編制相應(yīng)的濾波程序。
應(yīng)用單片機實現(xiàn)的心電數(shù)據(jù)采集器可以取代目前進口的高精度盒式數(shù)據(jù)記錄盒，并且可省略數(shù)據(jù)回送裝置，從而大大降低成本。它克服了磁帶式記錄器設(shè)計原理上的缺陷，通過硬件和軟件所采用的有關(guān)技術(shù)，顯著地改善了所采集的心電圖信號的品質(zhì)。實踐表明該智能心電數(shù)據(jù)采集器運行穩(wěn)定，能耗低，具有良好的應(yīng)用前景。
參考文獻
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