Effect of electrode impedance in improved buffer amplifier for bioelectric recordings

Abstract

We analysed the effects of electrode impedance on the transfer response of a one-stage improved buffer amplifier. The electrode DC resistance (R_d) modifies the one-stage buffer transfer response. We found a limit electrode resistance (R_d(lim)) which depends on the transfer damping factor (ϵ). If R_d is lower than 86.5 k Ω, the transfer response of the buffer fulfils American Heart Association (AHA) recommendations, but when R_d is greater than R_d(lim) it must be cautiously weighed up because its influence in the transfer response becomes appreciable. The maximum R_d that can be driven by the buffer is 1.2 M Ω. Higher values do not fulfil AHA recommendations. Therefore, electrodes with higher impedance should not be used with this kind of buffer. In contrast, 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.