This book presents fundamental requirements, electrical specification, and parameter tradeoffs of wearable EEG acquisition circuits, especially those compatible with dry electrodes for user-friendly recordings. The authors introduce active electrode, the most promising solution for dry electrodes-based EEG measurement. This architectural concept has been combined with various, innovative circuit design techniques to illustrate structured IC design methodologies for high performance EEG recording. This book also gives examples on the design, implementation and evaluation of three generations of active electrode ICs.@en

Active electrodes (AEs), i.e., electrodes with built-in readout circuitry, are increasingly being implemented in wearable healthcare and lifestyle applications due to AEs' 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, 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 an AE system interfaced with dry electrodes.@en

This paper presents a digital active electrode (DAE) system for multi-parameter biopotential signal acquisition in portable and wearable devices. It is built around an IC that performs analog signal processing and digitization with the help
of on-chip instrumentation amplifiers, a 12 bit ADC and a digital interface. Via a standard bus, up to 16 digital active electrodes (15-channels) can be connected to a commercially available microcontroller, thus significantly reducing system
complexity and cost. In addition, the DAE utilizes an innovative functionally DC-coupled amplifier to preserve input DC signal, while still achieving state-of-the-art performance: 60 nV/sqrt(Hz) input-referred noise and 350 mV electrode-offset tolerance. A common-mode feedforward scheme improves the CMRR of an
AE pair from 40 dB to maximum 102 dB.@en

This paper presents a theoretical analysis and measurements of the current noise of several chopper instrumentation amplifiers, which demonstrate that the charge injection and clock feed-through associated with the MOSFETs of the input chopper give rise to significant input current and current noise. In combi-
nation with high source impedances, this “chopper noise” is converted to voltage noise, which may then be a significant contributor to the amplifier’s total input-referred voltage noise. Chopper noise has a white power spectral density, whose magnitude is roughly proportional to the chopping frequency. Design guidelines are proposed to reduce chopper noise, as well as the use of a clock-boot-
strapped chopper, which generates significantly less noise than a traditional chopper.@en

This paper presents an active electrode system for gel-free biopotential EEG signal acquisition. The system consists of front-end chopper amplifiers and a back-end common-mode feedback (CMFB) circuit. The front-end AC-coupled chopper amplifier employs input impedance boosting and digitally-assisted
offset trimming. The former increases the input impedance of the active electrode to 2 G at 1 Hz and the latter limits the chopping induced output ripple and residual offset to 2 mV and 20 mV respectively. Thanks to chopper stabilization, the active electrode achieves 0.8 μVrms (0.5-100 Hz) input referred noise. The use of a back-end CMFB circuit further improves the CMRR of the active electrode readout to 82 dB at 50 Hz. Both front-end and back-end
circuits are implemented in a 0.18 μm CMOS process and the total current consumption of an 8-channel readout system is 88 μA from 1.8 V supply. EEG measurements using the proposed active electrode system demonstrate its benefits compared to passive electrode systems, namely reduced sensitivity to cable motion artifacts and mains interference.@en