BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 71

is equivalent to that of an inductor L R B .

For the values shown in the circuit, the equivalent inductance has an approximate value of
900 H. When this inductor is placed in series with a 10-nF capacitor (C1), ac signals reach-
ing the noninverting input of op-amp IC1A are shunted to ground via the series resistances
(R4

C R A R B in series with a loss resistance R R A

R5 R6 R7 R8) only for frequencies close to

1
2

1

2 53 Hz

π (9 0 0 H )( 1 0 n F )

π L C

The output of IC1A tracks the filter's input except for signals close to the notch. The depth

of the notch is controlled via R5 (lower value

fi

deeper notch), while the exact notch cen-

ter frequency can be trimmed via R6.
Yet another approach to the design of a notch fi filter is to combine a low- and a high-pass

fi filter that excludes only notch frequencies from its bandpass. You will recall

from our earlier discussion that a state-variable

filter to yield a fi

filter produces simultaneous high-pass,
low-pass, and bandpass outputs. Looking at the intersection of the low- and high-pass out-
puts of the state-variable

fi

fi filter shown in Figure 2.15, it is easy to see how an additional

op-amp con gured to sum the high-pass output with the low-pass output would yield a sig-

nal notched at the common cutoff

fi

ff frequency. The circuit of Figure 2.23 shows a notch fi filter

implemented using a Burr-Brown UAF42 state-variable filter IC. This IC incorporates

precision 1000-pF capacitors for the op-amp integrators and an auxiliary op-amp that is
used to sum the low- and high-pass outputs. As such, all that is needed to implement a
notch

fi

fi filter with this IC are fi five external resistors. The notch frequency is set via R1 and

R2 (where R1 R2) and is given by

1
1000 pF)

f notch 2

π (R1)(

Whatever notch filter will not

only remove the power line interference but will also take away parts of the signal of inter-
est. In addition, the notch

fi filter you chose to use, you must remember that the notch fi

fi filter may introduce nonlinear phase shifts in frequency compo-

nents within 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 standard
ECG signals since the main frequency components of P-, R-, and T-waves are far below
60 Hz. However, when ECGs are examined for small variations that are indicative of scar
tissue due to previous myocardial infarction, removal of power line interference has to be
done with utmost care not to eliminate or distort the ventricular late potentials , microvolt-
level (1 to 20

fi

V) waveforms that are continuous with the QRS complex, last into the ST
segment, and occupy a relatively wide frequency band (40 to 200 Hz) that peaks exactly
within the range 50 to 60 Hz.

µ

HARMONIC ELIMINATOR

Unfortunately, power line interference is not limited to 50 or 60 Hz. Fluorescent lights,
dimmers, and other nonlinearities introduce powerful components at the harmonics of the
power 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

fi

filter to eliminate power
line interference at the main frequency and its harmonics. However, an n-path

fi

fi filter is a

better way than this of implementing a comb filter implementation generates the

necessary poles by switching a sequence of capacitors in synchronism to the power line
fundamental frequency.

fi filter. This fi