A Bootstrapping Technique to Boost Input Impedance of ECG Recording Amplifiers

In recent years, the use of un-gelled electrodes has gained popularity in ambulatory ECG recording. More recently, textile-based electrodes have been explored in the context of wearable technology for telemedicine and remote monitoring. The input impedance of the amplifier considered in relation to that of the source electrodes has a profound effect on both the time and frequency domain responses of the amplifier as well as on its CMRR.This paper presents a single-supply ECG pre-amplifier that attains a balanced common-mode input impedance of 10 GΩ within a frequency range of 0.1-150Hz. Input capacitance is less than 50fF and the input-referred peak-to-peak noise voltage is less than 30µV. This is accomplished using an impedance boosting mechanism and a novel means of bootstrapping the power supply rails. Performance was verified using a prototype ECG amplifier implemented on a surface mounted PCB, which incorporated this front-end stage with additional standard instrumentation amplifier stages. The design offers a linear, low complexity circuit using low-power, low-noise commercially available op-amp, catering for a surface electrode impedance value as high as 10MΩ and meeting the IEC60601 performance specification while providing a CMRR greater than 90dB at mains frequency. Its power consumption is less than 75µW.