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2 mSEC > START NOISE (IF ENABLED) START PULSE- WIDTH TIMER PULSE WIDTH > 100 SEC FILTERS address some of the challenges facing designers of low-power, low-noise, highperformance, tethered or portable ECG systems. The AFE, designed for both monitor- and diagnostic-quality ECG measurements, comprises five electrode inputs and a dedicated right-leg-drive (RLD) output reference electrode. In addition to supporting the essential ECG signal-monitoring elements, the AFE enables such functions as respiration (thoracic impedance) measurement, lead/electrode connection status, internal calibration, and capabilities for pacing-artifact detection. One ADAS1000 supports five electrode inputs, facilitating a traditional, six-lead ECG measurement. By cascading a companion ADAS1000-2 device, the system can be scaled up to a true 12-lead measurement; by cascading three or more devices, the system can be scaled to measurements with 15 leads and beyond. detection algorithm The device’s front end includes a digital pacemaker artifact-detection algorithm that detects pacing artifacts with widths ranging from 100 μsec to 2 msec and amplitudes ranging from 400 μV to 1000 mV, to align with AAMI and IEC standards. Figure 5 is a flow diagram of the algorithm. The pace-detection algorithm runs three instances of a digital algorithm on three of four possible leads (I, II, III, or aVF). It runs on the high-frequency ECG data, in parallel with the internal decimation and filtering, and returns a flag that indicates pacing was detected on one or more of the leads, providing the measured height and width of the detected signal. For users who wish to run their own digital pace algorithm, the ADAS1000 supplies a high-speed pace interface that provides the ECG data at a 128-kHz data rate; the filtered and decimated ECG data remains unchanged on the standard interface. A minute-ventilation filter is built into the ADAS1000 algorithm. MV pulses, which are conducted from the ring of a bipolar lead to the housing of the pacemaker, detect respiration rates to control the pacing rate. They’re always less than 100 μsec wide, varying from about 15 to 100 μsec. The simultaneous three-vector pacing-artifact system can detect pacing artifacts in noisy environments. Each of the three instances of the pace algorithm can be programmed to detect pace signals on different leads (I, II, III, or aVF). Programmable threshold levels tailor the algorithm to detect the range of pulse widths and heights presented, with internal digital filters designed to reject heartbeat, noise, and MV pulses. When a pace has been validated in an individual instance of the pace signal, the device outputs a flag so that the user can mark or identify the pace signal in the ECG capture strip. The choice of sample rate for the pacing-artifact algorithm is significant because it cannot be exactly the same frequency as those used for the H-field telemetry carrier by the three pacingsystems companies (Boston Scientific, Medtronic, and St Jude). All three vendors use different frequencies, and each has many different telemetry systems. Analog Devices believes that the ADAS1000’s sampling frequency does not line up with that of any of the major telemetry systems.EDN References 1 National Academy of Engineering, “What is a pacemaker?” 2012, http://bit. ly/T4vXEH. 2 Fruitsmaak, Steven, “St Jude medical pacemaker with ruler,” image, 2007, Wikipedia, The Free Encyclopedia, http://bit.ly/V3KAJl. AuthorS’ biographies John Kruse is a field applications engineer for Analog Devices in Minneapolis. He joined ADI in 2005 and specializes in medical applications. He has authored many articles and patents; several of the patents cover pacing-artifact acquisition. Kruse graduated with a bachelor of science degree in electronics engineering from the University of Minnesota in 1980. In 1997, he received a master of science degree in electronics engineering from the University of St Thomas (St Paul, MN), where he currently is an adjunct professor. Catherine Redmond is an applications engineer at Analog Devices in Limerick, Ireland. Since joining ADI in 2005, she has gained industrial-market expertise by supporting precision DACs as applied in automatic test equipment. Redmond currently focuses on precision ADC products. She graduated from Cork Institute of Technology in Ireland with a bachelor’s degree in electronics engineering. START ENABLE PACE-DETECTION SELECT LEADS YES YES YES NO NO NO START PACE-DETECTION ALGORITHM LOOK FOR TRAILING EDGE TRAILING EDGE DETECTED? NOISE FILTER PASSED? FLAG PACE DETECTED UPDATE REGISTERS WITH WIDTH AND HEIGHT Figure 5 The flowchart shows the digitalpacemaker decision process for the artifact detection algorithm, which detects pacing artifacts with widths that range from 100 μsec to 2 msec and amplitudes that range from 400 μV to 1000 mV. w ww.edn-europe.com MARCH 2013 | EDN Europe 33


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