A Two-Wired High Input Impedance and High CMRR Active Electrode Insensitive to Component Mismatch

This article proposes a new two-wired amplifying active electrode (AE), presenting a high input impedance and a high common-mode rejection ratio (CMRR). In the proposed structure, the input impedance and the CMRR of the system are considerably increased by extracting the body common-mode (CM) signal and applying it to AEs. Moreover, the CMRR of the proposed structure is less sensitive to the mismatch existing between voltage gains of different AEs. In addition, the power-to-noise efficiency of the system is improved because the proposed circuit presents a high-voltage gain. A prototype is implemented using discrete components for electrocardiogram (ECG) recording, while the employed passive components are selected to have a voltage gain of 100 with 1% tolerance. The measurement results show that the proposed structure achieves a 107-dB CMRR at the fundamental power-line frequency (i.e., 50 Hz) with a 1.2- \text{M}\Omega source impedance unbalance. Moreover, it presents an input-referred noise voltage equal to 1.6~\mu \text{V}_{\text {RMS}} in a 0.5-100-Hz bandwidth and an input capacitance of only 2 fF.