An improved buffer for bioelectric signals

An AC-coupled amplifier that offers a high input impedance, thus making it suitable for bioelectric signal amplification, is discussed. The necessary formulas for calculating its input impedance and transfer function in order to facilitate its adaptation to different applications are presented. By using the appropriate equations, it is possible to choose the value of the output impedance, the maximum overshooting in the frequency response, and the low corner frequency. The effective input impedance is limited by common mode input capacitances associated not only with the operational amplifier, but also with input protections, RF filter, and circuit layout. These capacitances provide an increased impedance near frequency of resonance.<>