A two-step approach for EMI evaluation of a wearable ECG

In this study, we propose a two-step approach to evaluate electromagnetic interference (EMI) with a wearable electrocardiogram (ECG) sensor. The two-step approach combines a quasi-static electromagnetic (EM) field analysis and an electric circuit analysis, and is applied to the EMI analysis at frequencies below 1 MHz for our developed wearable ECG to demonstrate its usefulness. The quasi-static EM field analysis gives the common mode voltage coupled from the incident EM field at the ECG sensing electrodes, and the electric circuit analysis quantifies a differential mode voltage at the differential amplifier output of the ECG detection circuit. The differential mode voltage has been shown to come from a conversion from the common mode voltage due to an unbalance between the contact impedances of the two sensing electrodes. It may achieve nearly 0.6 V at the differential amplifier output under 10-V/m plane-wave incident electric field, and completely mask the ECG signal. It is essential to reduce the unbalance as much as possible to not cause a significant interference voltage in the amplified ECG signal.