Regarding the measurement of biopotentials, which one of the
following statements
is TRUE?
a) Signal-to-noise ratio must be minimised to optimise fidelity of displayed
biopotential
b) Electrocardiogram electrodes generate potential as well as
conduct current
c) Input impedance at the amplifier must be minimised in order
to maximise the
potential measured
d) The bandwidth of frequencies over which an electromyogram must
consistently
amplify is 0.5 to 100 Hz
e) Gain and common-mode rejection ratio are measured in Sone units
Answer: b
Explanation
Capacitance and inductance coupling as well as extrinsic
electromagnetic interference
are responsible for unwanted frequencies that disrupt the
transmission and reception
of biopotentials, which are small in magnitude. The use of
physical shields and
electronic filters serves to maximise signal-to-noise ratio to reduce
disruption of the
displayed potential. With electrocardiogram electrodes, the
combination of a metal
and its chloride (usually silver) making electrical contact with
tissues containing
electrolytes (which may conduct current) means that chemical
changes at the interface
may render the electrodes the equivalent of an electrochemical
cell. They may generate
their own potential and where conduction is available, current
will flow.
Electrodes
may become polarised as a result of this process. Input impedance
to an amplifier must
be large to avoid a step-down effect on the measured potential. It
is the electrode
impedance that must be small; 0.5 to 100Hz is the necessary
bandwidth for accurate
reception and display of an electrocardiogram. Electromyograms
have extremely
sharp spikes that require a very high frequency capability in
their processing because
Fourier analysis of the waveform reveals very high frequency harmonics.
The Sone is a
unit of perceived loudness. Sound intensity is often expressed in
decibels, which is also
the unit used to express gain and common-mode rejection ratio.
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