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Acquisition ASIC rolls for EEG systems

Posted: 13 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:EEG acquisition ASIC? clinical EEG systems?

(Click to view block diagram.)

IMEC rolled out a complete 200?W 8-channel EEG acquisition ASIC suitable for miniaturized ambulatory EEG measurement systems at the last International Solid State Circuit Conference. The ASIC meets the requirements for clinical EEG systems. Gain and filtering is programmable making it suitable for a wide range of ambulatory EEG applications.

IMEC's EEG acquisition ASIC comprises eight programmable readout channels, an 11bit ADC to digitize the amplified and filtered EEG signals, a square-wave oscillator to generate the 1MHz clock for the ADC and a bias circuit (see die diagram). Besides the acquisition function, it also comprises calibration of the readout channels and enables measurement of the impedance of the bio-potential electrodes.

The ASIC, fabricated in a 0.5?m CMOS process, measures 17.55mm? and consumes only 200?W. Such low power consumption makes it capable of operating for over a month from a standard coin cell battery like a CR2032, making it suitable for autonomous applications.

EEG signals are ?V-range low-frequency signals that suffer from a large amount of common-mode interference. Furthermore, the bio-potential electrodes that are needed to view the signals generate a non-negligible offset. To achieve signal extraction, the ASIC requires readout front-ends with high common-mode rejection ratio (CMRR), low-noise, and high-pass filter characteristics to filter the differential electrode offset voltage. The implemented front-end is based on AC-coupled chopper-stabilized instrumentation amplifiers with a DC-servo loop outside the choppers for the high-pass filtering. Each channel has adjustable high-pass filtering (cut-off at 0.1Hz) and the gain and bandwidth is electronically selectable. The front-ends achieve the lowest noise-efficiency factor (4.1) and the highest common-mode rejection ratio (CMRR >120dB) ever reported. The large input impedance (>1G) ensures high CMRR even if there is a mismatch between the electrode impedances. Meeting the critical IFCN (International Federation of Clinical Neurophysiology) requirements makes this ASIC applicable for clinical EEG systems.

The EEG ASIC is suitable for miniaturized ambulatory EEG acquisition systems which increase the patient's autonomy and quality of life. It's also applicable for sports devices, entertainment, comfort monitoring, and other health and lifestyle products and services. Interested parties can license the ASIC for further product development.

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