Biomedical Engineering Reference
In-Depth Information
ers are seldom built these days using individual op-amps. Instead, an
integrated circuit instrumentation ampli
Biopotential ampli
fi
er
(ICIA) combines in a single package most of
the components required to make an instrumentation ampli
fi
er. ICIAs typically require one
or two external resistors to set their gain. These resistors do not a
fi
ect the high CMRR
value or the high input impedance achieved in ICIAs through precise matching of their
internal components.
ff
Instrumentation Biopotential Amplifier
The circuit of Figure 1.27 is a typical ICIA-based biopotential ampli
er. This high-input-
impedance circuit combines a programmable-gain instrumentation ampli
fi
fi
er and an ac-
coupled (LPF:
3 dB at 0.5 Hz; HPF:
3 dB at 500 Hz) con
fi
gurable bandpass
fi
filter to
form a highly versatile,
compact,
stand-alone biopotential ampli
fi
er. Di
ff
erential
ampli
fi
cation of the biopotential signal is achieved with high CMR (
90 dB at 60 Hz)
through the use of a high-accuracy monolithic instrumentation ampli
fi
er IC. The low-noise
(
V
p-p
between 0.5 and 100 Hz) front end can be programmed to have a gain of 10,
100, or 1000, while a
1
ยต
fi
xed second stage and a con
fi
gurable third stage further amplify the
signal to an overall gain of up to 1 million.
Typical applications for this biopotential ampli
er
for standard and topographic EEG, evoked potential tests (BAER, MLAR, VER, SER),
and for cognitive signals and long-latency studies. The heart of the circuit is IC1, Burr-
Brown's INA102 programmable monolithic IC instrumentation ampli
fi
er are as a front-end and main ampli
fi
fi
er. Biopotentials
are dc-coupled to the instrumentation ampli
er through current-limiting resistors R1 and
R2. An INA102 gain of 1, 10, 100, or 1000 is selected by programming jumpers JP1 and
JP2 as shown in Table 1.1. Since the ampli
fi
fi
er is dc-coupled, care must be exercised in the
selection of gain so that the ampli
set voltages accompanying
the biopotential signal. For example, to use this circuit as part of a surface ECG ampli
fi
er is not saturated by dc of
ff
fi
er,
the gain must be calculated to cope with of
ff
set potentials of up to
300 mV.
The INA102 is ac-coupled (
3 dB at 0.5 Hz) to a second ampli
fi
cation stage with a
fi
xed
gain of 100. Resistor R5 and capacitor C2 form a low-pass
at
500 Hz. R6, R7, R8, C3, C4, and C5 are used to select the desired passband of two stages of
fi
fi
filter with
3-dB cuto
ff
filtering. R6-R8 and C3-C5, along with one-half of IC3, form a third-order (
18 dB/octave)
Butterworth low-pass active
fi
filter stage. The design of these
fi
filters is discussed in Chapter 2.
Finally, the ac output of the
fi
filter is presented to an inverting ampli
fi
er prior to output.
The gain of this last stage is given by
G
IC3B
R10/R9. As shown in Figure 1.28, compo-
nents R6-R9 and C3-C5 can be soldered onto a DIP header which is inserted in a 14-pin
DIP socket. A number of these DIP-header modules may be assembled to provide an
assortment of desired passband and gain characteristics. If the listed resistors and capaci-
tors are used, the low-pass
fixed at 22 Hz, with a third-stage gain of 10.
The supply voltage to the circuit must be symmetrical and within the range
3-dB point is
fi
5 V (min-
imum) to
16 V (absolute maximum). Rated speci
fi
cations are obtained using a supply of
15 V. Diodes D1 and D2 provide protection against incorrect supply voltage polarity, and
capacitors C7-C14 are used to decouple and
filter the power supply. Because of the very
small quiescent maximal supply current used by this circuit, a pair of 9-V alkaline batter-
ies constitute a suitable power supply for most applications. Preferably, leads to the bio-
electrodes should be low-loss low-capacitance coaxial cables whose shields are connected
to the subject ground terminal of the biopotential ampli
fi
fi
er. Construction of this biopoten-
tial ampli
er is simple and straightforward, but care must be taken to keep all wiring as
short and as clean as possible.
In using this biopotential ampli
fi
first stage low
(e.g., 10), and to reach the required overall gain by selecting a high gain for the third stage.
In addition, optimal rejection of unwanted signal components is best achieved by careful
fi
er it is desirable to keep the gain of the
fi
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