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Abstract-Active Electrodes (AE), i.e. electrodes with built-in readout circuitry, are increasingly being implemented in wearable healthcare and lifestyle applications due to AE's robustness to environmental interference. An AE locally amplifies and buffers µV-level EEG signals before driving any cabling. The low output impedance of an AE mitigates cable motion artifacts thus enabling the use of high-impedance dry electrodes for greater user comfort. However, developing a wearable EEG system, with medical grade signal quality on noise, electrode offset tolerance, common-mode rejection ratio (CMRR), input impedance and power dissipation, remains a challenging task. This paper reviews state-of-the-art bio-amplifier architectures and low-power analog circuits design techniques intended for wearable EEG acquisition, with a special focus on AE system interfaced with dry electrodes. Index Terms-Active electrode, instrumentation amplifier (IA), electroencephalography (EEG), dry electrodes, common-mode rejection ratio (CMRR), brain-computer interface (BCI)I. INTRODUCTION ecent advances in biomedical technologies, integrated circuits (ICs), sensors and data analysis techniques have accelerated the development of wearable technology for Telehealth applications. Today, miniature and low-power medical sensors can be easily integrated into various accessories that continuously sense, process and transfer people's physiological information during their daily life activities. By reducing the need for manual intervention and by lowering the cost, these medical devices are being widely used in personal healthcare and home diagnostics, such as wellness and health monitoring, home rehabilitation, and the early detection of brain disordersElectroencephalography (EEG)

This paper describes an 8-channel gel-free EEG/electrode-tissue impedance (ETI) acquisition system, consisting of nine active electrodes (AEs) and one back-end (BE) analog signal processor. The AEs amplify the weak EEG signals, while their low output impedance suppresses cable-motion artifacts and 50/60 Hz mains interference. A common-mode feed-forward (CMFF) scheme boosts the CMRR of the AE pairs by 25 dB. The BE post-processes and digitizes the analog outputs of the AEs, it also can configure them via a single-wire pulse width modulation (PWM) protocol. Together, the AEs and BE are capable of recording 8-channel EEG and ETI signals. With EEG recording enabled, ETIs of up to 60 kΩ can be measured, which increases to 550 kΩ when EEG recording is disabled. Each EEG channel has a 1.2 GΩ input impedance (at 20 Hz), 1.75 µVrms (0.5-100 Hz) input-referred noise, 84 dB CMRR and ±250 mV electrode offset rejection capability. The EEG acquisition system was implemented in a standard 0.18 µm CMOS process, and dissipates less than 700 µW from a 1.8 V supply.

We realized design solutions to enhance the photoresponsive performance of self-powered TiO2UV photodetectors by employing Ag nanowires as metal contacts.