Biomedical Engineering Reference
In-Depth Information
C21, and C23 is used to decouple the positive input power rail of IC6 so that switching
noise within IC6 does not
fi
find its way into the postisolation circuits.
SCRATCH, RUMBLE, CLICK, AND POP
Our good British friend and colleague André Routh has held many positions as a biomed-
ical engineer and scientist at various cardiac pacemaker and diagnostic device companies
for over 20 years. In the tradition of British electronics a
, espe-
cially when it's done using valves (vacuum tubes). André's expertise is in biomedical sig-
nal processing, and some time ago he implemented a specialized
fi
cionados, his hobby is hi-fi,
fi
fi
filter for cardiac signals
based on the way in which audio professionals
fi
filter surface noises from old records
(remember vinyl?). In the hi-fi
fifi
field, these circuits are known as scratch or “click and pop”
fi
filters. Another device, the rumble
fi
filter, cuts down low-pitched noises, such as vibration
from the phonograph motor.
The circuit of Figures 2.36 to 2.39 is a high-pass
fi
filter with a cutoff
ff
frequency of 0.5 Hz
for baseline of
ff
set rejection and a low-pass
fi
filter with a cutoff
ff
frequency of 500 Hz for
noise rejection and antialias
filter unit can be varied continuously
in three ranges, from 0 to a maximum of 100. What makes this
fi
filtering. The gain of this
fi
filter so unique is its rapid
recovery from transient overloads. This is achieved through an input “blanking” scheme
that can reject impulses such as pacing spikes. The high-pass
fi
filter with its long time con-
stant can cause output saturation when faced with a step-type input or slow decay process
such as a de
fi
brillation shock. An output overload detection circuit changes the time con-
stant of the high-pass
fi
frequency is increased to approximately 50 Hz until
the output is within the allowable range for the signal acquisition equipment that follows
the
fi
filter. The cutoff
ff
filter.
Typically, this
fi
fi
filter circuit would be used between an isolated biopotential ampli
fi
er
and an instrumentation tape recorder or data acquisition unit. The output of the
filter is
monitored on a LED bar graph display which has a VU meter response with fast attack and
slow decay times. Additionally, there are two LEDs to indicate either input or output over-
loads. In a typical application, an input signal arrives via J2 from the output of an isolated
biopotential ampli
fi
er. R1, D1, and D2 form an input signal clamp that limits at approxi-
mately 8.2 V. R1 and C1 form a low-pass
fi
frequency of 5.3 kHz. R2 is a
passive attenuator whose wiper is connected to the noninverting input of IC1A. The gain
of this op-amp is de
fi
filter with a cutoff
ff
ned by R3 and R4. R4 should be selected to have a resistance equal
to R2, so that when R2 is set to maximum the voltage at IC1A's output is equal to the input
voltage at J2.
IC1B and associated components form a second-order Butterworth (maximally
fi
fl
at
amplitude) low-pass
fi
filter. With the components selected, the cutoff
ff
frequency calculated
is 509 Hz. Front-panel SW1 can be used to bypass the low-pass
filter. IC2 is a quad CMOS
transmission gate (digitally controlled analog switch). Under normal conditions (i.e., with
no input overload), IC2A is closed and IC2B is open, allowing the signal at the wiper of
switch SW1 to feed the second-order Butterworth high-pass
fi
fi
filter built around IC1C
(Figure 2.37). With the components selected, the cutoff
frequency is 0.49 Hz. As long as
there is no output overload, analog switches IC2C and IC2D are open, and as such, under
normal conditions they do not interfere with the nominal operation of the high-pass
ff
fi
filter.
Front-panel switch SW2 can be used to bypass the high-pass
fi
filter.
IC1D is con
fi
gured as a switchable gain ampli
fi
er. The gains available are
1,
10, and
100, depending on the setting of front-panel switch SW3. The low-pass action of C6
depends on the gain of the ampli
fi
er stage. At
100 the cuto
ff
frequency is approximately
4.8 kHz; at
10 it is about 53 kHz. Potentiometer R13 in conjunction with resistor R14
allows the output o
ff
set to be nulled. R18 and C7 form a low-pass
fi
filter with a cutoff
ff
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