Design and Development of Medical Electronic Instrumentation

Chapter 1 - Biopotential Amplifiers

In general, signals resulting from physiological activity have very small amplitudes and
must therefore be amplified before their processing and display can be accomplished. The
specifications and lists of characteristics of biopotential amplifiers can be as long and confusing
as those for any other amplifier. However, for most typical medical applications, the
most relevant amplifier characterizing parameters are the seven described below.

  1. Gain. The signals resulting from electrophysiological activity usually have amplitudes on
    the order of a few microvolts to a few millivolts. The voltage of such signals must be amplified
    to levels suitable for driving display and recording equipment. Thus, most biopotential
    amplifiers must have gains of 1000 or greater. Most often the gain of an amplifier is measured
    in decibels (dB). Linear gain can be translated into its decibel form through the use of
 Gain(dB) = 20 log10(linear gain)
  1. Frequency response. The frequency bandwidth of a biopotential amplifier should be
    such as to amplify, without attenuation, all frequencies present in the electrophysiological
    signal of interest. The bandwidth of any amplifier, as shown in Figure 1.1, is the difference
    between the upper cutoff frequency f2 and the lower cutoff frequency f1. The gain at these
    cutoff frequencies is 0.707 of the gain in the midfrequency plateau. If the percentile gain
    is normalized to that of the midfrequency gain, the gain at the cutoff frequencies has
    decreased to 70.7%. The cutoff points are also referred to as the half-power points, due to
    the fact that at 70.7% of the signal the power will be (0.707)2 = 0.5. These are also known
    as the -3-dB points, since the gain at the cutoff points is lower by 3 dB than the gain in
    the midfrequency plateau: -3dB = 20 log10(0.707).
  2. Common-mode rejection. The human body is a good conductor and thus will act as
    an antenna to pick up electromagnetic radiation present in the environment. As shown in
    Figure 1.2, one common type of electromagnetic radiation is the 50/60-Hz wave and its
    harmonics coming from the power line and radiated by power cords. In addition, other
    spectral components are added by fluorescent lighting, electrical machinery, computers,
    and so on. The resulting interference on a single-ended bioelectrode is so large that it often
    obscures the underlying electrophysiological signals.

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