An Analog Front-End Chip With Self-Calibrated Input Impedance for Monitoring of Biosignals via Dry Electrode-Skin Interfaces

This paper demonstrates an input impedance boosting method that was developed for long-term monitoring of electroencephalography signals. An instrumentation amplifier was designed with a negative capacitance generation feedback (NCGFB) technique to cancel the adverse effects of input capacitances from electrode cables and printed circuit boards. The NCGFB boosts the measured impedance from below 40 M<inline-formula> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> to above 500 M<inline-formula> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> at 50 Hz when the equivalent capacitance at the inputs is up to 150 pF. The prototype chip includes an automatic calibration system to adaptively enhance the input impedance through on-chip test signal generation, measurement, and the automatic digital control of the NCGFB. Consisting of an instrumentation amplifier, a low-pass notch filter, and a variable gain amplifier in 130-nm CMOS technology, the signal path has a combined gain range of 66–93 dB with a total power consumption of 42 <inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math></inline-formula>W. The front-end bandwidth covers 0.5–48 Hz, and its integrated input-referred noise over the bandwidth is 3.75 <inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math></inline-formula>V<sub>rms</sub>. The measured third-order harmonic distortion component is at least 57 dB below the fundamental signal level. A common-mode rejection ratio of 77.6 dB and a power supply rejection ratio of 74 dB were measured at 10 Hz. When activated, the auxiliary test signal generation and calibration circuits consume a power of 542 <inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math></inline-formula>W.

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