Biomedical Engineering Reference
In-Depth Information
be powered from batteries or through a properly rated isolation power supply. The same
isolation requirements apply to the output of the ampli
fi
er.
BOOTSTRAPPED AC-COUPLED BIOPOTENTIAL AMPLIFIER
Direct ac coupling of the instrumentation ampli
fi
er's inputs by way of
RC
high-pass
fi
filters
across the inputs degrades the performance of the ampli
fi
er. This practice loads the input
of the ampli
fi
er, which substantially lowers input impedance and degrades the CMRR of
the di
ers can be used to present a high-
input impedance to the biopotential source, any impedance mismatch in the ac coupling of
these to an instrumentation ampli
ff
erential ampli
fi
er. Although unity-gain input bu
ff
fi
er stage degrades the CMR performance of the biopo-
tential ampli
er.
Suesserman has proposed an interesting modi
fi
fi
cation of the standard biopotential instru-
mentation ampli
er that retains all of the supe-
rior performance inherent in dc-coupled instrumentation ampli
fi
er to yield an ac-coupled di
ff
erential ampli
fi
er designs. The circuit of
Figure 2.6 is described by Suesserman [1994] in U.S. patent 5,300,896. If capacitors C3 and
C4 were not present, the circuit of Figure 2.6 would be very similar to that of the ac-coupled
instrumentation ampli
fi
er described earlier in the chapter. ICIA IC1 without C3 and C4
would be ac-coupled to the biopotential signal via capacitors C1 and C2. Just as in the earlier
ac-coupled biopotential ampli
fi
fi
er, resistors R1, R2, R3, and R4 are needed to provide a dc
path to ground for the ampli
er's input bias currents. In this circuit, these resistors would
limit the ac impedance of each input to 2 M
fi
R4) referred to ground.
With C3 and C4 as part of the circuit, however, ac voltages from the outputs of the
ICIA's di
Ω
(R1
R2 and R3
ff
erential input stage are fed to the inverting inputs of their respective ampli-
fi
fiers. This causes the ac voltage drop across R1 and R4 to be virtually zero. Ac current
fl
ow
through resistors R1 and R4 is practically zero, while dc bias currents can
flow freely to
ground. This technique is known as
bootstrapping
, referring allegorically to the way in
which the ampli
fl
er nulls its own ac input currents, as when one pulls his or her own boot-
straps to put boots on.
Since bootstrapping capacitors C3 and C4 almost completely eliminate ac current
fi
ow
through R1 and R4, the input current through ac-coupling capacitors C1 and C2 would also
drop close to zero, which by Ohm's law translates into an almost in
fl
fi
nite input impedance
(since
R
V
/
i
;
R
tends to
∞
as
i
approaches 0). Suesserman described this biopotential
ampli
er as having an impressive 120-dB CMRR (at 100 Hz) with an input impedance of
more than 75 M
fi
Ω
.
PASSIVE FILTERS
The simplest
filters contain
some combination of resistive (
R
), capacitive (
C
), and inductive (
L
) elements. The inductive
and/or capacitive components are required because these elements present varying imped-
ance to ac currents at di
fi
filters are those that comprise only passive components. These
fi
erent frequencies. As a refresher, you may remember that inductive
reactance increases with frequency, whereas capacitive reactance decreases with frequency.
Most passive
ff
filters used in the processing of biopotential signals are the resistive-capacitive
or
RC
kind. This is because relatively large and heavy inductors would be required to
implement
fi
filters at the low-frequency bands where biopotential signals reside, making
inductive-capacitive (
LC
)
fi
filters impractical.
Despite their simplicity,
RC
fi
ective in processing a wide
variety of biopotential signals. Take, for example, the complete biopotential ampli
fi
filters are very common and e
ff
fi
er
Search WWH ::
Custom Search