Accessing Differential Measures with a Conjugate Coil-Pair for Wireless Resistive Analog Passive (WRAP) ECG Sensors

Abstract

Wearable devices and mobile-based biomedical applications have increased the demand of finding an alternative easy-to-use method for monitoring body signals. One approach towards this challenge is a zero power (battery-less), fully-passive electronic patch sensors that can be attached to the body. We have previously reported a novel technique for Wireless Resistive Analog Passive (WRAP) sensors for various body signals such as core body temperature, heart rate, etc. This passive sensor consists of a loop antenna, a tuning capacitor, and resistive-based biopotential transducer. The wireless scanner transmits an RF signal at frequency 8.37 MHz, which is amplitude modulated based on the resistive changes by a transducer at the WRAP sensor. The envelope of the modulated signal represents the body signal to be captured and can be analyzed on the scanner or downstream on the user's smartphone. In this work, we first demonstrate the capability for differential signal capture, such as electrocardiogram (ECG or EKG) using a novel conjugate coil pair technique. The WRAP sensor uses a dual-gate MOSFET (depletion mode) to convert the biopotential signal to the correlated resistive variation of the source to drain resistance. These two circuits power the conjugate coil pair, which cancels common mode signal and only transmits differential mode signals (viz. ECG). The results show that connecting a pair of sensors in this way could allow accurate measurement of a differential biopotential. This work demonstrates voltage sensitivity down to $40 \boldsymbol{\mu} \mathbf{V}$ towards realizing a battery-less, body-worn WRAP ECG sensor for monitoring ECG signals.