Imec (Leuven, Belgium) and the Holst Centre (Eindhoven, The Netherlands) have developed an analog-signal processor application-specific integrated circuit (ASIC) that results in a fivefold reduction in the overall power consumption of ambulatory heart-activity signal-monitoring systems. This is a major step toward producing autonomous wireless sensor systems that constantly monitor patients' health in order to diagnose chronic illness, according to the institutes. By continuously processing biopotential signals from electrocardiographs (ECGs) or by transmitting raw data over a wireless link, digital signal processors (DSPs) or radios in today's biomedical wireless sensor systems waste a great deal of power. In addition, current ambulatory biomedical sensor systems suffer from motion artifacts that affect the robustness of detection algorithms and demand even more processing power. The new ASIC developed in Imec and the Holst Centre's Human++ program uses an adaptive sampling scheme based on activity detection. This technology reduces the amount of data that must be processed by the DSP or transmitted by radio. By preprocessing the signal, a simplified DSP can be used for accurate R-peak detection, reducing the DSP's power consumption. Motion artifacts are detected by continuously monitoring electrode-tissue impedance. Impedance monitoring, in turn, can also be used for ensuring signal integrity by continuously checking the electrode connectivity. Operating at 2 V, the ASIC consumes only 30 µW of power. It consists of an ECG readout channel, two quadrature readout channels for continuous-time monitoring of electrode-tissue impedance, and two quadrature readout channels for tracking signal fluctuations in a specific frequency band. The component also includes an activity detector that senses the frequency content of the ECG signal and adapts the sampling rate of the integrated analog-to-digital converter to digitize the ECG signal. The ASIC stage results in an elevenfold reduction in power consumption of the DSP and a sixfold reduction in wireless transmission, reducing the overall power consumption of the wireless heart-activity signal-monitoring system below 300 µW. This budget also includes the power required for continuous-time electrode-tissue contact impedance measurement. "Within the Human++ program, Imec and Holst Centre develop solutions for an efficient and better healthcare," remarks Bert Gyselinckx, general manager of Imec the Netherlands at the Holst Centre. "Self-powered intelligent body area networks with wireless sensors promise to be a solution for more comfortable healthcare systems. This breakthrough is a major step towards constant ambulatory monitoring of people using energy harvesting, which increases the comfort level of patients and is a cost- and time-efficient alternative for current monitoring systems."
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