BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 73

The simpli fi ed circuit of Figure 2.24 can be used to explain the process by which such

a filter operates. Here, eight capacitors are switched to ground in synchronism with the

power line, making this circuit an eight-path

fi

filter. A power line sample obtained from
transformer T1 and coupled through R8 and C14 is fed to a 74HC4046 phase-locked loop
(PLL). A PLL consists of a phase comparator, a loop

fi

filter, and a voltage-controlled oscil-
lator. In this application, the phase comparator compares the phase and frequency of the
power line sample against those of the internal oscillator and adjusts the oscillator fre-
quency so that it equals some exact multiple of the incoming reference. A 74HC161 is
clocked at 16 times the power line frequency by the output of the PLL's oscillator, gener-
ating eight digital addresses addresses (on outputs QB-QD) that divide the power line
cycle evenly into eight segments. A 74HC4051 is driven by the digital sequence, selecting
which one of eight capacitors is connected to ground throughout the power line cycle.
During the time that one such capacitor is connected to ground, it samples the

fi

fi filter's input

signal (the biopotential signal to be comb- ltered, not the power line reference) with a time

constant given by R2. The output of the eight-path

fi

filter is inverted by IC1C and added to
the original input signal to yield the comb response. Please note that IC1D is used to gen-
erate a virtual ground at 2.5 V. However, a split power supply would work equally well by
substituting ground by

fi

2.5 V, 2.5 V by ground, and 5 V by 2.5 V.

An intuitive explanation of the comb fi filter's transfer function is that the capacitors of the

n -stage erence between the current signal voltage and the

voltage integrated over the same time segment on previous power line cycles. Each capac-
itor can be assumed to store an average of the signal at the speci

fi filter charge to a portion of the di ff

c time segment of the
power line cycle. Since the components of interest in the signal to be

fi

filtered are in most
cases uncorrelated to the power line, the capacitors store only a sample of signal compo-
nents locked to the power line frequency (i.e., the power line fundamental and its harmon-
ics along with any other correlated noise). The output signal is then cleaned from repetitive
power line noise when the power line-locked average is subtracted from the input signal.
This type of

fi

fi filter has unique advantages over fi fixed-frequency notch fi filters. First, the

fi filter automatically adapts the frequency of its notches to whatever power line frequency

is present at the reference port. Second, the ect the signal when no inter-

ference is present. Last, power line-frequency biopotential signal components not locked
to the power line are not a

fi filter does not a ff

filter excludes only signals that maintain a
phase lock to the power line for a number of power line cycles. In addition, unlike the
notch

ff ected since this fi

fi filters described earlier, changes in component values in this circuit have minimal

e ff ect on the fi filter's response, making it maintenance free.

filter is not a continous-time system. The use of switched capacitors makes this a
sampled system that is bound by Nyquist's sampling theorem. This limits the theoretical
bandwidth of the

This fi

filter to one-half the sampling frequency. Since the signal is sampled
eight times during the power line cycle, the theoretical bandwidth of the

fi

filter is four times
the reference frequency, making it possible to reject only the fundamental,

fi

rst, and sec-
ond harmonics (60, 120, and 180 Hz for a 60-Hz reference). Higher bandwidth with rejec-
tion of higher harmonics requires increasing the number of sampling capacitors. For
example, a 16-path

fi

fi filter at 60 Hz would have a theoretical bandwidth of 480 Hz.

filter of Figure 2.24 needs additional components to reject more noise than
that which it introduces by its switching action. This is usually accomplished through
various bandpass

The comb fi

fi filters placed at the output of the n -stage fi filter as well as at the output

of the summing ampli er. A family of ready-made universal eliminator modules based

on this principle is available from Electronic Design & Research Inc. Models EDR-82534
and EDR-82534A are adaptive comb

fi

cally for integration within
medical instrumentation. These modules comprise a 64-path

fi filters designed speci fi

filter and support a signal
bandwidth of dc to 500 Hz (EDR-82534) or dc to 1200 Hz (EDR-82534A). Figure 2.25
shows the pinout and connection to the module. The module's internal block diagram is

fi