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1 引言在生物电记录过程中,医学护理的安全规则限定了在病人接触大地或电源情况下的最大泄漏电流。虽然国与国间的安全规则不尽相同,但是通常都要求充分的隔离,以及小于800pF的隔离电容(当病人接近220V、50Hz电线时,最大泄漏电流为50μA)。一个小于300pF的隔离电容就能够保证,病人的情况不会因为与记录设备相接而发生很大变化,一般人体对地的电容也在这个数量级。2 隔离共模电压的来源典型测量环境及其所有相关电容见图1(开关断开)。人体与电源间电容C_(body)引起电流的i_(amp)从参比电极(Zrl,ECG的右腿电极)流过放大器的隔离公共端,再经隔离电容(Ziso)到地,因此,隔离两端就存在一个电压——隔离共模电压(Vim,后同)。在所有的隔离放大器中,都有一部分Vim馈通到输出端。放大器抑制Vim的能力是以隔离共模抑制比(简称IMRR,后同)来衡量的: IMRR=20lg((Vim·A)/Vout) (1)其中Vim——隔离共模电压Vout——Vim馈通引起的放大器输出信号A——放大器的总增益本文将记叙两种防止Vim造成输出干扰信号的方法:一种是用附加电路减小隔离电容两端的实际电压;另一种方法更为优越,即设计一种放大器,使得高Vim的馈通忽略不计,具体方法见4部分。
1 Introduction During bioelectric recording, safety rules for medical care define the maximum leakage current when a patient contacts the earth or power supply. Although the safety rules vary from country to country, adequate isolation and isolation capacitances of less than 800pF are usually required (maximum leakage current of 50μA for patients approaching 220V, 50Hz wire). An isolation capacitor of less than 300pF ensures that the patient’s condition does not change significantly due to contact with the recording device, and the typical body-to-ground capacitance is also on the order of magnitude. 2 Sources of Isolated Common-Mode Voltage A typical measurement environment and all its associated capacitances are shown in Figure 1 (switch open). The body-to-body power C_ (body) causes the current i_ (amp) to flow from the reference electrode (Zr1, the right leg of the ECG) through the isolated common of the amplifier and through the isolation capacitor (Ziso) There is a voltage at both ends - isolated common mode voltage (Vim, the same). In all isolation amplifiers, a portion of Vim is fed to the output. The ability of an amplifier to reject Vim is measured in terms of isolated common-mode rejection ratio (IMRR for short): IMRR = 20lg ((Vim · A) / Vout) where Vim - Isolated Common Mode Voltage Vout - Vim Amplifier Output Signal Caused by Feedthrough A - Total Amplifier Gain Two ways to prevent Vim from producing output disturbances are described in this article: one is to use additional circuitry to reduce the actual voltage across the isolation capacitor; the other is more advantageous , Design an amplifier that neglects the high Vim feedthrough. See Section 4 for details.