Ultra-High Input Impedance Buffer for Dry or Capacitive Electrodes: Design and Characterization for Industry

A large number of health and wellness applications wish to monitor electrocardiogram signal (ECG) in the least intrusive way possible. The use of dry textile electrodes with high input impedance bio-amplifiers is a common technique. This paper presents a novel ultra-high input impedance buffer to obtain a reduced input capacitance and enhance the acquisition of results for dry or non-contact capacitive ECG electrodes. Based on different bootstrapping techniques, input stage biasing and unipolar power supply, an operational amplifier buffer is implemented on Printed Circuit Board (PCB) to provide a low cost solution. The solution considers the protections for electrostatic discharges (ESD) and filters for interferences, maintaining high input impedance design. A method to measure input impedances in the range of femtofarads is also proposed. This active electrode prototype presents a 191 fF input capacitance at 50 Hz. Different skin-electrode impedance cases are studied to obtain the Common Mode Rejection Ratio (CMRR). As a result, a value of 81.4 dB is obtained with zero mismatch and 76.1 dB at 50 Hz is obtained with a resistive mismatch of 1–2.2
$\text{M}\Omega $
. Moreover, ECG test is evaluated using a discrete back-end circuit, digital filtering and software display. Finally, the full ECG circuit has been simulated under an ESD transient of ±8 kV IEC 61000–4-2 waveform. This paper demonstrated that the proposed solution with standard discrete components fulfills the industry requirements for robust ultra-high impedance ECG sensor.