28.6 A 22.6µ W Biopotential Amplifier with Adaptive Common-Mode Interference Cancelation Achieving Total-CMRR of 104dB and CMI Tolerance of 15Vpp in 0.18µm CMOS

Abstract

Improving robustness to common-mode interference (CMI) is imperative for reliable two-electrode ECG recording. CMI degrades the signal quality in two ways. First, it appears as a differential-mode signal together with the ECG, because of common-mode to differential-mode (CM-DM) conversion. This is often due to the electrode impedance mismatch [1], as contacts inevitably lose their properties differently during long-term recording. Second, CMI that exceeds the supply voltage saturates the amplifier and distorts the recording when the subject is near power lines [4]. To address both of these issues, the recoding IC must withstand large CMI and also have a large total-CMRR (T-CMRR) that includes the effect of electrode mismatch. Unfortunately, prior works provide solutions to only one of them, and thus there is a lack of reliable long-term ECG recording in a practical setting. In [2, 3], CM-DM conversion is addressed by increasing the input impedance in common-mode. However, the IA fails when there is large CMI. In addition, one of the main issues of the input impedance boosting method is that chopping, despite its benefit in 1/f noise reduction, cannot be used, as it reduces the input impedance. Hence, a CM-DM suppression technique that is effective for low input impedance IA is desired. In [4, 5], tolerance to CMI is improved by absorbing the CMI current. However, their T-CMRRs are poor $(\sim65$ dB). In this work, we propose a CMI-tolerant amplifier with an adaptive CMI-canceling technique. It ensures $\gt100$ dB of T-CMRR when there is an electrode mismatch of $152k \Omega$ and CMI of 15V, even when the input capacitance is as large as 120pF.