Effect of the Electrode Impedance in Improved Buffer Amplifier for Bioelectric Recordings

Abstract

We analyzed the effects of the electrode impedance on the transfer response of a one-stage improved buffer amplifier. The electrode DC resistance (Rd) modifies the one-stage buffer transfer response. We found a limit electrode resistance (Rd(lim)) which depends on the transfer damping factor (∈). If Rd is lower than 86.5 kW, the transfer response of the buffer fulfils the American Heart Association, AHA, recommendations, but when Rd is greater than Rd(lim) it must be cautiously weighted-up because its influence in the transfer response becomes appreciable. The maximum Rd that can be driven by the buffer is 1.2 MW. Higher values never fulfil the AHA recommendations. Therefore, electrodes with higher impedance should not be used with this kind of buffer. On the other hand, when this buffer is used to build-in an instrumentation amplifier (IA) for bipolar recording, the common-mode rejection ratio (CMRR) is sensitive to the electrode type used.