Examples of Special Analog Circuits and Systems

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A critical part of the design was the acoustic resistance, Rac, across which the sound pressure drop was assumed to be without phase shift over the operating frequency range of the instrument, i.e., Rac was real over the frequency range. Many “pure” acoustical resistances, such as those used in pneumotachs, etc., are made from many parallel capillary tubes. Capillary tubes’ acoustic resistance is real up to some frequency at which they begin looking inductive due to the acoustic inertness of the tubes (Olson, 1940). To extend the range of real Rac, a stack of (parallel) thin slits with rectangular cross sections was used. It can be shown (Northrop, 2002) that the Rac of slits remains real to a frequency significantly higher than that for an equivalent Rac made from capillary tubes. This RAIMS system was designed to operate from 0.3 to 300 Hz. An earlier system described by Pimmel et al. (1977) that used a commercial capillary-tube pneumotach for Rac had a high frequency limit of 16 Hz before the pneumotach turned significantly reactive.

The prototype RAIMS system described here worked well with phantom acoustic lung impedances and normal volunteers in the lab, but was not investigated clinically.

12.9 Chapter Summary

Four diverse examples were chosen to illustrate the use of analog electronic circuit ICs in biomedical instrumentation system design:

1.The microdegree polarimeter represents an instrument that has evolved from a manually nulled instrument, with limited sensitivity to the optical rotation angle caused by polarized light passing through an optically active medium, to a closed-loop optoelectronic system with microdegree sensitivity. The optical rotation is used to measure D-glucose concentration in clear liquids. In the author’s design, an expensive Faraday rotator was eliminated; instead the water solvent of the test solution was used for the Faraday medium.

2.Also developed was a closed-loop laser velocimeter and ranging system in which a CW laser beam was sinusoidally amplitude modulated (instead of using pulsed laser light and measuring nanosecond delays in the return reflection). The frequency of the amplitude modulations was automatically adjusted so that the phase difference between the transmitted and received modulations was held constant. This LAVERA system gave two simultaneous analog outputs proportional to target range and velocity; it was designed as a prototype aid for blind persons.

3.The third system described was a self-balancing impedance plethysmograph designed to measure small changes in volume as changes

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538	Analysis and Application of Analog Electronic Circuits

in admittance in certain anatomical regions, such as the chest or legs. This system operated at a constant 75 kHz and used feedback to null an error voltage between the applied voltage and a voltage proportional to the body part’s admittance at a standard condition. It was used to detect respiration and heartbeat simultaneously in an experimental context.

4. The fourth system was a respiratory acoustic impedance measurement system (RAIMS) — a prototype instrument intended to detect obstructive lung disease by comparison with “normal” records. The acoustic driving point impedance was defined and the acoustic equivalent of the voltmeter–ammeter method of measuring electrical impedance was described. An acoustic pressure source (analogous to a voltage) from a loudspeaker forced acoustic volume flow through a real acoustic resistance and then the unknown acoustic impedance of the respiratory system (pharynx, trachea, bronchial tubes, and alveoli). Specially modified microphones were used to sense the driving-point pressure at the mouth, P2, and the volume

flow proportional to pressure difference to across the acoustical resis-

· − tance, Q2 = (P1 P2)/Rac. Instrument display was in polar form: Zac(f)

vs. –Zac(f) over 0.3 ≤ f ≤ 300 Hz.

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