esistance of the piezoelectric sensor's amplifier should amounts to few meg-aohms. Input current should be at the level of nanoamperes. It makes provisions for the use of amplifiers with field transistors.
2) Noise characteristics of amplifiers with field transistors are marked by a high level of low-frequency flicker noises prevailing in the operating range. Therefore, acceptable results can be obtained by means of JFET transistors with the level of noise of no more than 1 dB, e.g., 2SK170, 2SK209, 2SK2394, or 2SK3557, and chosen couples for formation of bipolar circuits (2SK2145 or 2SK3320). Speaking about low-noise operating amplifiers, it is advisable to use ADA4004-1 and AD8671.
3) The construction of acoustic sensors implies hard contact with patients' skin; therefore, it is necessary to use medical stainless steel under the 18/10 brand or analogues as a metal membrane the piezoceramic element is attached to.
4) Hard contact between the sensor and patient's skin significantly lowers down requirements to the sensor's acoustic design. The only condition is a need for relatively strong attachment of the piezoelement's membrane to its body. On the one hand, it creates additional conditions supporting high-quality contact with the skin. On the other hand, it reduces the level of external noises and interference.
These activities were performed within the framework of State Contract No. 14.579.21.0019 concluded between the Ministry of Education and Science of the Russian Federation and LLC Diagnostika + on the subject "Development of the device for 24-h monitoring over mother and fetus' state of health during pregnancy by means of controlling parameters of the cardiovascular system based on acoustic data". In addition, the authors express their heartfelt gratitude to National Research Tomsk State University for the provision of premises and necessary equipment to conduct independent research for the purpose of choosing the type of sensors that allow detecting acoustic signals of the human bronchopulmonary system. This independent review is continuation of the authors' work performed under State Contract No.2013-1.4-14-514-01 concluded between the Ministry of Education and Science of the Russian Federation and LLC Diagnostika + on the subject "Development and research on the methods of producing informational images of bronchial sonography signals and algorithms to detect pathological signs for more objective diagnostics of bronchopulmonary diseases in pediatrics".
[1.] Kovacs F, Horvath C, Balogh AT, Hosszu G (2011) Extended noninvasive fetal monitoring by detailed analysis of data measured with phonocardiography. IEEE Transactions on Biomedical Engineering 1(58): 64-70.
[2.] Sovijarvi A, Vanderschoot J, Earis J (2000) Computerized respiratory sound analysis (CORSA): Recommended standards for terms and techniques. European Respiratory Review 77(10): 585-649.
[3.] Sovijarvi AR, Vanderschoot J, Earis JE (2000) Standardization of computerized respiratory sound analysis. European Respiratory Review 77(10): 585-588.
[4.] Torok M, Kovacs F, Seres S, Bartos I, Szekely I, etal. (1995) Method and Apparatus for Measuring Fetal Heart Rate and an Electroacoustic Sensor for Receiving Fetal Heart Sounds. US Patent No. 6245025 B1-8P
[5.] Pronichev IV (2004). Lectures on Physiology of Central Nervous System. Moscow: Swift, p. 214.
[6.] Veytsenfeld A (2000) Microphones. Zvukorezhisser 1: 4-8.
[7.] Itskovich AI, Shumarova EYu, Korenbaum VI (2005) Modern problems of respiratory sound analysis. Pacific Medical Journal 2: 11-13.
[8.] ADXL103/ADXL203 Precision [+ or -] 1.7 g, [+ or -] 5 g, [+ or -]18 g Single-/Dual-Axis iMEMS Accelerometer (2004) Analog Devices, p. 16.
[9.] ENDEVCO Devices for Measurement of Vibrations and Impulse Loads (2011) ENDEVCO Co., p. 16.
[10.] Ayoob Khan TE, Vijayakumar P (2010) Separating heart sound from lung sound using LabView.
International Journal of Computer and Electrical Engineering 3(2): 1793-8163.
[11.] Zuckerwar AJ, Mowrey DL (2003) Passive Fetal Heart Monitoring System. US Patent No. 6551251 B2-14 P
Publ