BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 41

BANDPASS SELECTION FOR
BIOPOTENTIAL AMPLIFIERS

As shown in Table 2.1, common biopotential signals span the range dc to 10 kHz. Under
ideal conditions, a biopotential ampli er with wideband response would serve most appli-

cations. However, the presence of common-mode potentials, electrode polarization, and
other interfering signals often obscure the biopotential signal under investigation. As such,
the frequency response of a biopotential ampli

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fi er should be tuned to the speci fi c spectral

content expected from the application at hand.
Spectral analysis is the most common way of determining the bandwidth required to
process physiological signals. For a first estimate, however, the rigors of spectral analysis

can be avoided simply by evaluating the durations of high- and low-frequency components
of the signal. Koide [1996] proposed a method for estimating the

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3-dB bandpass based

on acceptable distortion.
The duration of the highest-frequency component, t HF , is estimated from a stereotypical
signal to be the minimum rise or fall time of a signal variation. The duration of the lowest-
frequency component, t LF , on the other hand, is measured from the tilt of the baseline or
of the lowest-frequency component of interest. Koide illustrated this with an example.
Figure 2.1 shows a stereotypical intracellular potential measured from the pacemaker cells
in a mammalian heart SA node. In this example, t HF 75 ms and t LF 610 ms. Using the

formulas of Table 2.2, the ampli 3-dB bandpass of 0.0026 to

41.3 Hz to reproduce the signal with negligible distortion (1%). Acceptable distortion,
usually considered to be 5% or less for physiological signals, would require a narrower

fi cation system must have a

3-dB bandpass, of 0.013 to 18.7 Hz.

WIDEBAND BIOPOTENTIAL AMPLIFIER

The biopotential ampli er circuit described by the schematic diagrams of Figures 2.2 and

2.3 covers the complete frequency range of commonly recorded biopotentials with high
CMR. In this circuit, a Burr-Brown INA110AG ICIA is dc-coupled to the electrodes via
current-limiting resistors R22 and R23. Two Ohmic Instruments IS-1-3.3DP semiconductor

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