Biomedical Engineering Reference
In-Depth Information
m
V
20
Time
(s)
0
0
1.0
1.5
0.5
-20
610 ms
-40
-60
75 ms
Figure 2.1
A stereotypical intracellular potential measured from the pacemaker cells in a mam-
malian heart SA node has a minimum rise time of
t
HF
75 ms and a tilt of
t
LF
610 ms. The
3-dB
bandpass needed to reproduce this signal with 1% distortion is of 0.0026 to 41.3 Hz.
TABLE 2.2
3-dB Frequencies Required for the Reproduction
of Physiological Signals with Negligible and Acceptable Levels of Distortion
Approximate
3-dB Point
Negligible Distortion (1%)
Acceptable Distortion (5%)
0
t
.
0
6
0
(
1
s)
0
t
L
.0
F
(
0
s
8
)
High-pass (Hz)
L
F
3
.
1
s)
1
.
4
s)
Low-pass (Hz)
t
H
t
H
(
(
F
F
IC5 is powered via a Burr-Brown ISO107 isolation ampli
fi
er IC3, which generates
isolated
12 V when powered from
12 V. L1, C6, C7, and C8 form a pi
fi
filter to clean
the isolated
12-V power line generated by the ISO107 from switching noise. An identi-
cal network is used to
filter formed by L3, C15,
C16, and C17 is used to decouple the positive input power rail of IC3 so that switching
noise within IC3 does not
fi
filter the negative isolated supply rail. A pi
fi
fi
find its way into the postisolation ampli
fi
er stages (IC2 and IC4).
R10 and C21 form a low-pass
3dB of approximately 7 kHz to eliminate any
remaining trace of the carrier used to convey the signal across IC3's isolation barrier.
Op-amp IC2B is con
fi
filter with
er can
be selected through potentiometers R16-R20 that are switched via SW3. These provide
di
fi
gured as a noninverting ampli
fi
er. The gain of this ampli
fi
ff
erent levels of feedback to IC2B, depending on their setting. The output of this ampli-
fi
of approximately
7 kHz to eliminate any residual switching noise that may have coupled through the input
or power supply and ampli
fication stage is
fi
filtered via R9 and C20 with a
3dB low-pass cutoff
ff
fi
ed by IC2B. The ampli
fi
ed signal is bu
ff
ered and further low-
pass
fi
filtered via
fi
fixed-gain noninverting ampli
fi
er IC4 before being presented to the output
connector J1.
Since the biopotential ampli
er is dc-coupled and there are occasions when the signal
of interest may have a relatively large input dc of
fi
ff
set, an automatic zero-o
ff
set circuit has
been implemented through IC2A. Whenever o
set nulling is desired, momentary push-
button switch SW1 should be pressed. Doing so presents a sample of the output signal to
the integrating sample-and-hold (S&H) circuit formed by IC2A, R5, and C9. The hold capac-
itor should be a low-leakage type, and the path between the capacitor and the inverting
ff
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