Biomedical Engineering Reference
In-Depth Information
As such, this capacitor is practically a short circuit for RF currents, while it leaves low-
frequency signals pass unimpeded.
R5, R6, R9, R10, and C22 implement a low-pass
filter. Because of the virtual-ground
property governing the inputs of an op-amp, we can assume that the low-pass character-
istics of this
fi
resistor (either R5 and R6 in series, or R9 and
R10 in series) and the 470-pF capacitor. The
fi
filter are given by a 94-k
Ω
3-dB cuto
ff
frequency for an
RC
low-pass
fi
filter is
1
RC
f
3dB
2
π
which provides the input network with a
fi
first-order low-pass cutoff
ff
of approximately
3.6 kHz.
The biopotential ampli
fi
er's main low-pass
fi
filters are implemented by two cascaded
RC
passive
ers the signals between the cas-
caded sections. Both
RC
sections are identical, therefore setting a pole at the same fre-
quency. However, the e
fi
filters. An op-amp unity-gain follower (IC4) bu
ff
ect of the second
RC
can be suppressed by disconnecting its
capacitor through switch SW2. When SW2 is open, signals at the output of IC4 are fed to
unity-gain bu
ff
ff
er IC6 through R11. Since the input impedance of IC6 is practically in
fi
nite,
R11 has no e
ect on the signal. However, when SW2 is closed, R11 and the capacitor
selected by SW1B form a low-pass
ff
fi
filter. The nominal cutoff
ff
frequencies that can be
selected for the second-order
fi
filter were selected to be close to 1, 2, 5, 10, 20, 50, 100, 200,
and 500 Hz. The exact
3-dB cuto
ff
frequencies are shown in Table 2.3.
filters are implemented in essentially the same way as the low-pass sec-
tions. In Figure 2.8, however, the
RC
elements are reversed. Each high-pass section has a
capacitor (C50 and C53) which opposes current
The high-pass
fi
flow with an impedance that varies
inversely with frequency, and a resistor of selectable value that shunts the load. Both
RC
sections are identical, therefore setting a pole at the same frequency. However, the e
fl
ff
ect
of the second
RC
can be suppressed by shorting C53 through SW5. Op-amp IC13 bu
ff
ers
the signal between the stages. The nominal cutoff
ff
frequencies that can be selected for the
second-order
fi
filter were selected to be close to 1, 2, 5, 10, 20, 50, 100, 200, and 500 Hz.
The exact
3-dB cuto
ff
frequencies are shown in Table 2.4.
filter is implemented using a LTC1152 instead
of a UPC4250 op-amp as in the case of the other followers because higher current output
is required to drive the lowest resistor values associated with the highest
The
fi
first follower (IC13) in the high-pass
fi
3-dB cuto
ff
TABLE 2.3 Low-Pass
3-dB Cutoff Frequencies for the Biopotential
Amplifier of Figure 2.7 Selected through SW1
a
3-dB Cutoff Frequency for
3-dB Cutoff Frequency for
Second-Order Low-Pass Filter
First-Order Low-Pass Filter
SW1 Position
(Hz) (SW2 Closed)
(Hz) (SW2 Open)
1
1.02
1.59
2
1.83
2.84
3
4.65
7.23
4
10.22
15.92
5
21.75
33.86
6
46.47
72.34
7
102.2
159.2
8
217.5
338.6
9
464.7
723.4
a
SW2 selects between
fi
rst- or second-order response.
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