Determining the input impedance of ECG amplifiers using accurate electrode modelling

The influence of the skin-electrode-amplifier interface and the input impedance of ECG recording amplifiers on transient response performance is investigated using accurate modeling of surface contact electrodes. The paper aims to establish the input impedance requirements of ECG recording amplifiers based on the electrical properties of the electrodes.

The IEC 60601 standard stipulates the performance requirements for electrocardiographs. Analyses and simulation of both dc and ac modes of coupling the electrodes to the input of the amplifier have been undertaken using an accurate double time-constant model of the electrodes in order to establish design constraints for amplifier input impedance to meet this performance specification. These have been backed up with bench tests.

Investigations carried out indicate that the minimum 10 MΩ resistance at the amplifier input recommended in the specification is insufficient when using some modern adhesive electrodes and increasingly popular un-gelled or dry electrodes. Design constraints are established based on the electrical properties of the electrodes. These constraints suggest that the input impedance of the amplifier should be as high as 10 GΩ and the amplifier ac coupled cut-off frequency should not to be higher than 0.05 Hz for all the electrode models considered.

Signal distortions in the form of false S-wave creation and depression of the S-T segment have been observed when the previously recommended 10 MΩ input impedance is used in the amplifier. This distortion can lead to clinical misdiagnosis but can be avoided if the design constraints established in the paper are adopted.