Biomedical Engineering Reference
In-Depth Information
er of Figure 2.4, dc and very low frequency potentials are pre-
vented from propagating beyond the front-end ampli
In the biopotential ampli
fi
er through a technique commonly
referred to as dc rejection . In the circuit, signals picked up by electrodes attached to the
patient's skin are dc-coupled and ampli
fi
fi
ed by IC1, a Burr-Brown INA110 instrumentation
ampli
er IC. The gain of the front-end stage is programmable between unity and 500 by
jumpers JP11-JP14. Potentiometer R17 is used to trim the input o
fi
set to IC1. Since IC1
is dc-coupled, care must be exercised in the selection of gain so that the ampli
ff
fi
er is not
saturated by dc o
set voltages accompanying the biopotential signal. For example, to use
this circuit as part of a surface ECG ampli
ff
fi
er, the gain must be calculated to cope with
o
300 mV. In general, IC1's gain should be kept low so that
dc-coupling does not result in its saturation.
To reject dc, IC4C together with R11 and C17 are used to To
ff
set potentials of up to
set IC1's reference to
suppress a baseline composed of components in the range dc to 0.48 Hz. Once the dc com-
ponent is removed, the dc-free biopotential signals are ampli
ff
ed via IC4A and IC4B.
Notice that we used clipping diodes at the inputs and feedback paths of this speci
fi
c imple-
mentation. Our application involved measuring the small electrical response of cardiac
cells after the delivery of large stimuli. If you build this circuit, you may chose to leave
D4-D7 and D9-D12 out of the circuit.
Galvanic isolation is provided by IC2, a Burr-Brown ISO107 isolation ampli
fi
er IC. In
addition to providing a signal channel across the isolation barrier, the ISO107 has an inter-
nal dc-dc converter which powers the isolated side of the ISO107 circuitry as well as pro-
viding isolated power (
fi
15 mA typical) for the rest of the circuitry of the isolated
front end (i.e., IC1 and IC4). The output gain of IC2 is selected through jumpers JP4-JP6
to provide gains of 1, 10, or 100. IC3's output is then low-pass
15 V at
fi
filtered by IC3.
AC-COUPLED INSTRUMENTATION BIOPOTENTIAL
AMPLIFIER FRONT END
The circuit of Figure 2.5 embodies the classic implementation of a medium-impedance
(10-M
er based on the popular AD521 ICIA by
Analog Devices. The gain of this circuit is adjustable between 10 and 1000 and maintains
a CMR of at least 110 dB. This circuit of
) instrumentation biopotential ampli
fi
ff
ers superior dynamic performance with a mini-
mal ac-coupled signal bandwidth (
3 dB) of 40 kHz and low noise (1
µ
V p-p at G
10,
0.1-100 Hz). This circuit is an example of a biopotential ampli
er front end suitable for
recording EMG or ECG signals or as a general-purpose high-impedance ac-coupled trans-
ducer ampli
fi
er.
The heart of the circuit is IC1, the monolithic IC instrumentation ampli
fi
fi
er. Biopotentials
are ac-coupled to the ampli
fi
er's inputs through C1 and C2. Although instrumentation ampli-
fi
er's
input. As such, resistors R1 and R2 are required to provide a dc path to ground for the ampli-
fi
fiers have di
ff
erential inputs, bias currents would charge stray capacitances at the ampli
fi
fier's input bias currents. These resistors limit the impedance of each input to 10 M
referred
to ground. The high-pass
filter, formed by the ac-coupling capacitor and the bias shunt resis-
tor on each of the ICIA's inputs, has a
fi
frequency of 0.12 Hz.
The gain of IC1 is determined by the ratio between R3 and R4. Using a 20-k
3-dB cuto
ff
multi-
turn potentiometer, and given that the value of the range-setting resistor R3 is 100 k
, the
di
ff
erential gain of the ampli
fi
er can be trimmed between 5 and 1000. The output of
ff
set of
the ampli
fi
er can be trimmed through R5, which, at any given gain, introduces an output
o
ff
set equal and opposite to the input o
ff
set voltage multiplied by the gain. Thus, the total
output o
ff
set can be reduced to zero by adjusting this potentiometer. The instrumentation
ampli
fi
er provides a low-impedance output (0.1
) with a permissible swing of
10 V and
can source or sink up to 10 mA.
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