Biomedical Engineering Reference
In-Depth Information
TABLE 2.4 High-Pass
3-dB Cutoff Frequencies for the Biopotential
Amplifier of Figure 2.8 Selected through SW4
a
3-dB Cutoff Frequency for
3-dB Cutoff Frequency for
Second-Order High-Pass Filter
First-Order High-Pass Filter
SW4 Position
(Hz) (SW5 Open)
(Hz) (SW5 Closed)
1
1.03
0.66
2
2.06
1.33
3
5.27
3.39
4
10.32
6.63
5
20.65
13.26
6
48.59
31.21
7
103.3
66.31
8
206.5
132.6
9
485.9
312.1
a
SW5 selects between first- or second-order response.
frequencies when SW5 is closed. Figure 2.9 shows the notch
fi
filters that are used to
fi
lter
power line interference. More detail will be presented on notch
fi
filters later in the chapter.
Su
filter (built around IC15 and IC17) has a notch at around
50 Hz, while the other (built around IC16 and IC18) has a notch at 60 Hz. Trimmers R59
and R60 are used to
ce it to say for now that one
fi
fi
fine-tune the notch frequency, while R57 and R58 select the notch
depth.
The circuit of Figure 2.10 can be made to process the output signal coming out
of the notch
fi
filters. This circuit performs full-wave recti
fi
cation on the input signal.
Zero-threshold recti
fi
cation is achieved by placing the recti
fi
er diodes (D6) within the
feedback loop of op-amp IC11. Full-wave recti
fi
cation results from adding an inverted
half-wave-recti
fi
ed signal at double amplitude to the original signal in the summing
ampli
cation is an operation that is often used when the desired
information can be extracted by analyzing the energy conveyed by the biopotential sig-
nal. For example, the EMG signal is often recti
fi
er IC12. Full-wave recti
fi
ltered to yield a
signal proportional to the force generated by a muscle. The full-wave recti
fi
ed and then low-pass-
fi
fi
er can be
bypassed through SW3.
The signal is then bu
ered by IC10 in Figure 2.11 before it is optically isolated from
recording instruments connected at output connector J2. Galvanic isolation ensures that the
source of biopotential signals (e.g., a patient) is not exposed to dangerous currents leaked
from power lines through the subject's heart. This function is implemented through a
Hewlett-Packard HCNR201 high-linearity analog optocoupler. This optocoupler includes
one LED and two photodiodes, the output photodiode and an input photodiode designed
to receive the same light intensity from the LED as the output photodiode. The LED cur-
rent is controlled through a feedback loop so that the current at the input photodiode is
proportional to the voltage of the analog signal at the input of IC10. Under the hypothesis
that both photodiodes receive the same light intensity, the current at the output photodiode
also follows the input analog signal.
Op-amp IC8 and capacitor C43 integrate the di
ff
erence between the current through R18
and the current through the input photodiode. The output of this integrator drives the LED
so that these currents are equalized. Therefore, the current through the input photodiode
(which will be equal to the current of the output photodiode) is equal to the input analog
signal divided by R18. The analog signal is recovered at the output through the bandwidth-
limited current-to-voltage converter implemented by IC9, R15, and C44. The output of the
integrator of the input stage drives the LED through a constant-transconductance stage
implemented by Q1, R16, R14, R13, and R17.
ff
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