Biomedical Engineering Reference
In-Depth Information
4. Noise and drift. Noise and drift are additional unwanted signals that contaminate a
biopotential signal under measurement. Both noise and drift are generated within the
ampli
er circuitry. The former generally refers to undesirable signals with spectral
components above 0.1 Hz, while the latter generally refers to slow changes in the baseline
at frequencies below 0.1 Hz.
The noise produced within ampli
fi
fi
er circuitry is usually measured either in microvolts
peak to peak (
µ
V p-p ) or microvolts root mean square (RMS) (
µ
V RMS ), and applies as if it
were a di
erential input voltage. Drift is usually measured, as noise is measured, in micro-
volts and again, applies as if it were a di
ff
erential input voltage. Because of its intrinsic low-
frequency character, drift is most often described as peak-to-peak variation of the baseline.
5. Recovery. Certain conditions, such as high o
ff
ff
set voltages at the electrodes caused by
movement, stimulation currents, de
brillation pulses, and so on, cause transient interrup-
tions of operation in a biopotential ampli
fi
fi
er. This is due to saturation of the ampli
fi
er
caused by high-amplitude input transient signals. The ampli
fi
er remains in saturation for a
fi
finite period of time and then drifts back to the original baseline. The time required for the
return of normal operational conditions of the biopotential ampli
fi
er after the end of the
saturating stimulus is known as recovery time .
6. Input impedance. The input impedance of a biopotential ampli
fi
er must be
su
ciently high so as not to attenuate considerably the electrophysiological signal under
measurement. Figure 1.3 a presents the general case for the recording of biopotentials.
Each electrode-tissue interface has a
finite impedance that depends on many factors, such
as the type of interface layer (e.g., fat, prepared or unprepared skin), area of electrode sur-
face, or temperature of the electrolyte interface.
In Figure 1.3 b , the electrode-tissue has been replaced by an equivalent resistance net-
work. This is an oversimpli
fi
cation, especially because the electrode-tissue interface is not
merely a resistive impedance but has very important reactive components. A more correct
representation of the situation is presented in Figure 1.3 c , where the
fi
fi
final signal recorded as
the output of a biopotential ampli
er is the result of a series of transformations among the
parameters of voltage, impedance, and current at each stage of the signal transfer. As shown
in the
fi
fi
figure, the electrophysiological activity is a current source that causes current
fl
ow i e
in the extracellular
fluid and other conductive paths through the tissue. As these extracellu-
lar currents act against the small but nonzero resistance of the extracellular
fl
fl
fluids R e , they
produce a potential V e , which in turn induces a small current
ow i in in the circuit made up
of the reactive impedance of the electrode surface X Ce and the mostly resistive impedance
of the ampli
fl
first stage, the currents from each of the bipo-
lar contacts produce voltage drops across input resistors R in in the summing ampli
fi
er Z in . After ampli
fi
cation in the
fi
fi
er,
where their di
finally produce an output voltage V out .
The skin between the potential source and the electrode can be modeled as a series
impedance, split between the outer (epidermis) and the inner (dermis) layers. The outer
layer of the epidermis—the stratum corneum—consists primarily of dead, dried-up cells
which have a high resistance and capacitance. For a 1-cm 2 area, the impedance of the stra-
tum corneum varies from 200 k
ff
erence is computed and ampli
fi
ed to
fi
at 1 Hz down to 200
at 1 MHz. Mechanical abrasion
will reduce skin resistance to between 1 and 10 k
at 1 Hz.
7. Electrode polarization. Electrodes are usually made of metal and are in contact with
an electrolyte, which may be electrode paste or simply perspiration under the electrode.
Ion-electron exchange occurs between the electrode and the electrolyte, which results in
voltage known as the half-cell potential . The front end of a biopotential ampli
er must be
able to deal with extremely weak signals in the presence of such dc polarization components.
These dc potentials must be considered in the selection of a biopotential ampli
fi
fi
er gain, since
they can saturate the ampli
er, preventing the detection of low-level ac components.
International standards regulating the speci
fi
fi
c performance of biopotential recording systems
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