Go to DBLP homepageA 4.6μW 3.3-NEF Biopotential Amplifier with 133VPP Common-Mode Interference Tolerance and 102dB Total Common-Mode Rejection Ratio for Two-Electrode Recording System
Abstract: Physiological data, such as EEG and ECG, are crucial in delivering vital information for medical diagnostics and research applications. Recently, the demand for biopotential recording using two electrodes has grown thanks to its better user experience and lower cost than counterparts with three electrodes [1], [2]. However, a two-electrode recording IC suffers from a large common-mode interference (CMI) over $100\mathrm{V}_{\text{PP}}$ [3], potentially saturating an analog front-end (AFE) or resulting in large CM to differential-mode (DM) conversion. These challenges necessitate biopotential AFEs to possess a large CMI tolerance as well as a high total common-mode rejection ratio (T-CMRR) while providing excellent noise efficiency with low power consumption. Figure 15.7.1(a) depicts a simplified electrical model of CMI coupling from a power source $(\mathrm{V}_{\text{PO}})$ in a two-electrode recording system attached to a human body [4]. In a ground-isolated system, a displacement current $(\mathrm{l}_{\mathrm{d}})$ splits into $\mathrm{l}_{\mathrm{b}}$ and $\mathrm{l}_{\text{GND}}$, which flow through $\mathrm{C}_{\text{Body}}$ and $\mathrm{C}_{\text{GND}}$, respectively. The high CM input impedance of the IA $(\mathrm{Z}_{\text{lN}-\text{CM}-\mathrm{C}})$ converts the $\mathrm{l}_{\text{GND}}$ into a large CMI voltage $(\mathrm{V}_{\text{CMl}-\mathrm{C}})$ relative to the chip ground. The $\mathrm{C}_{\text{GND}}$ represents the parasitic capacitance between floating chip ground and earth ground, which ranges from 1 to $3\text{pF}$ depending on the size of a recording IC and battery [4]. CMI cancellation techniques based on the CM charge pump (CMCP) in Fig. 15.7.1(b) can tolerate CMI up to $20\mathrm{V}_{\text{PP}}$ and achieve high T-CMRR [1], [2], [5], [6]. However, these approaches introduce significant noise $( > 2\mu \mathrm{V}_{\text{rms}})$ and increase power consumption due to the CMCP, rendering them unsuitable for EEG measurements. An alternative method, the CM averaging unit (CMAU) reported in [7], reduces CMI by driving the chip ground to match the CMI. However, this design overlooks the $\mathrm{C}_{\text{GND}}$, making them feasible only for specific systems without the parasitic $\mathrm{C}_{\text{GND}}$.
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