Pallás-Areny R, Webster J G
Departamento de Ingenieria Electrónica, U.P.C., Barcelona, Spain.
Ann Biomed Eng. 1990;18(3):251-62. doi: 10.1007/BF02368441.
We present the design of an instrumentation biopotential amplifier that, (a) combines the ac coupling and high input impedance of an ac-coupled buffer with the CMRR of a simple differential amplifier or a monolithic instrumentation amplifier, (b) improves the CMRR by using a potentiometer without requiring either precision resistors or high-CMRR op amps, (c) illustrates how to calculate the CMRR for differential-input stages for either differential output or single-ended output. With our approach, for the 741 op amp, the CMRR of the differential stage exceeds 126 dB at 10 Hz and 106 dB at 100 Hz, which is higher than the 90 dB of the op amp alone. This is because a potentiometer permits mismatch of the resistor ratio, which compensates for the low CMRR of the op amp. Use of the LF355 op amp, which has a wider band-width, yields a CMRR of 126 dB at both 10 and 100 Hz. If the second stage is an instrumentation amplifier then no adjustment is necessary. Mismatch of passive components in the bootstrapped input buffer stage decreases the CMRR from 126 dB to 112 dB at 60 Hz and decreases it further at lower frequencies, but that is not a problem for the usual power-line interference.
我们展示了一种仪器生物电位放大器的设计，该放大器：(a) 将交流耦合缓冲器的交流耦合和高输入阻抗与简单差分放大器或单片仪器放大器的共模抑制比（CMRR）相结合；(b) 通过使用电位计提高CMRR，而无需精密电阻器或高CMRR运算放大器；(c) 说明如何计算差分输入级对于差分输出或单端输出的CMRR。采用我们的方法，对于741运算放大器，差分级在10 Hz时的CMRR超过126 dB，在100 Hz时为106 dB，高于运算放大器单独时的90 dB。这是因为电位计允许电阻比失配，从而补偿了运算放大器的低CMRR。使用具有更宽带宽的LF355运算放大器，在10 Hz和100 Hz时的CMRR均为126 dB。如果第二级是仪器放大器，则无需调整。自举输入缓冲级中无源元件的失配会使60 Hz时的CMRR从126 dB降至112 dB，在更低频率时进一步降低，但这对于通常的电力线干扰来说不是问题。
