Time-Domain Response of ECG Bioelectric Amplifiers

This paper investigates the low-frequency response of a multi-stage bioelectric amplifier intended for use in the measurement of the electrocardiogram (ECG) using dry contact electrodes. These electrodes have an impedance which is typically an order of magnitude greater than that of the standard disposable self-adhesive electrodes used in clinical ECG recording. The design was undertaken with the intention of exploiting micropower CMOS operational amplifier technology to minimise power in ambulatory recording. The response is optimised to meet the transient response requirements of the International Electrotechnical Commission Standard 60601 applying to electrocardiographs [1,2]. The optimum configuration was established to be two differential stages with a gain of 20dB each and a differential-to-single-ended output stage having unity gain. The -3dB pole is placed at 0.013Hz in the first and second stages to give an overall -3dB low cut-off frequency of 0.02Hz. In addition, a zero at 0.0013Hz in the non-inverting front-end stage was cancelled by the pole of the input ac coupling network. This ensured that the maximum undershoot of 100μV and the maximum recovery slope of 300μVs permitted in response to a narrow pulse of 3mV amplitude and 100ms duration were met. Key-Words: ECG Recording, Bioelectric Amplifiers, Dry Electrodes, Un-gelled Electrodes.