DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

HARMONIC ELIMINATOR 71is equivalent to that of an inductor L CR A R B in series with a loss resistance R R A R B .For the values shown in the circuit, the equivalent inductance has an approximate value of900 H. When this inductor is placed in series with a 10-nF capacitor (C1), ac signals reachingthe noninverting input of op-amp IC1A are shunted to ground via the series resistances(R4 R5 R6 R7 R8) only for frequencies close to1 2π LC12π(900H)( 10nF)53 HzThe output of IC1A tracks the filter’s input except for signals close to the notch. The depthof the notch is controlled via R5 (lower value deeper notch), while the exact notch centerfrequency can be trimmed via R6.Yet another approach to the design of a notch filter is to combine a low- and a high-passfilter to yield a filter that excludes only notch frequencies from its bandpass. You will recallfrom our earlier discussion that a state-variable filter produces simultaneous high-pass,low-pass, and bandpass outputs. Looking at the intersection of the low- and high-pass outputsof the state-variable filter shown in Figure 2.15, it is easy to see how an additionalop-amp configured to sum the high-pass output with the low-pass output would yield a signalnotched at the common cutoff frequency. The circuit of Figure 2.23 shows a notch filterimplemented using a Burr-Brown UAF42 state-variable filter IC. This IC incorporatesprecision 1000-pF capacitors for the op-amp integrators and an auxiliary op-amp that isused to sum the low- and high-pass outputs. As such, all that is needed to implement anotch filter with this IC are five external resistors. The notch frequency is set via R1 andR2 (where R1 R2) and is given by1f notch 2π(R1)( 1000 pF)Whatever notch filter you chose to use, you must remember that the notch filter will notonly remove the power line interference but will also take away parts of the signal of interest.In addition, the notch filter may introduce nonlinear phase shifts in frequency componentswithin the filter’s passband.Take, for example, applications that require very subtle analysis of the ECG signal.Arbitrary removal of power line frequency signals may not pose a problem for standardECG signals since the main frequency components of P-, R-, and T-waves are far below60 Hz. However, when ECGs are examined for small variations that are indicative of scartissue due to previous myocardial infarction, removal of power line interference has to bedone with utmost care not to eliminate or distort the ventricular late potentials, microvoltlevel(1 to 20 µV) waveforms that are continuous with the QRS complex, last into the STsegment, and occupy a relatively wide frequency band (40 to 200 Hz) that peaks exactlywithin the range 50 to 60 Hz.HARMONIC ELIMINATORUnfortunately, power line interference is not limited to 50 or 60 Hz. Fluorescent lights,dimmers, and other nonlinearities introduce powerful components at the harmonics of thepower line frequency. A number of independent notch filters at 60, 120, 180 Hz, and so on(or their 50-Hz counterparts), could be cascaded to yield a comb filter to eliminate powerline interference at the main frequency and its harmonics. However, an n-path filter is abetter way than this of implementing a comb filter. This filter implementation generates thenecessary poles by switching a sequence of capacitors in synchronism to the power linefundamental frequency.