DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

SLEW-RATE LIMITING 81identification and removal. A good example of this are the artifacts produced by cardiacpacemakers on the ECG signal. The Association for the Advancement of Medical Instrumentation(AAMI) recommends designing ECG equipment assuming the ECG waveformof Figure 2.30. The highest slew rate for an ECG can be estimated by dividing the maximumpeak amplitude within the AAMI range of 0.5 to 5 mV and dividing it by the minimum risetime of the QR interval within the AAMI range of 17.5 to 52.5 ms. This gives a maximumslew rate of 5 mV/17.5 ms 0.28 V/s for a worst-case ECG pulse. The slew rate of apacing pulse is much higher than this, making it possible to design a slew-rate filter thatlimits the rate of change in the signal rather than a specific frequency band.This slew-rate filter technique is also useful in other applications that require limitingartifacts from fast transients. For example, it can be used to filter large artifacts producedby magnetic-resonance imaging (MRI) equipment, allowing a patient’s vital signs to bemonitored during procedures involving this imaging technique. Removing “pop” artifacts(fast transients caused by movement) from EEG recordings and limiting stimulus artifactsin devices designed to measure nerve conduction are also possible through the use of slewratelimiters.A simple slew-rate limiting filter is shown in Figure 2.31. This filter, designed by Williams[1998], is a simple op-amp buffered RC low-pass filter modified by the addition of a bidirectionaldiode clipping network. Whenever the input voltage to op-amp IC1 differs from itsoutput voltage by at least one forward diode drop, the forward-biased diodes will conduct.Under these conditions, the voltage at the output and noninverting input of the op-amp areequal, causing the voltage across R2 to remain approximately constant at one forward diodedrop, V F DIODE .A constant voltage across R2 forces a constant current that charges C1 linearly rather thanexponentially. If R2 R1, the maximum slope (∆V out /∆time) of the signal at the output isgiven by∆VoutVFDIODEslope max ∆ time ( R2)(C1)Input signals with a slope higher than this cause a constant rate of change at the outputequal to this limit. A signal segment with a slope lower than this limit passes through thea r0.5 mV max½ sinewavea sad TTd = 37.5 ms mind QTFigure 2.30 The AAMI stereotype ECG waveform has a maximum slew rate of 0.28 V/s, which allows separating the ECG’s signalcomponents from artifacts with higher slew rates, such as those from pacing and MRI.