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A Reconfigurable Multichannel Capacitive Sensor Array Interface

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5 Author(s)
Kyriakis-Bitzaros, E.D. ; Dept. of Electron., Technol. Educ. Inst. of Piraeus, Egaleo, Greece ; Stathopoulos, N.A. ; Pavlos, S. ; Goustouridis, D.
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The design of a reconfigurable capacitive sensor array interface comprised of a subsystem able to read an array of capacitive-type sensors and an embedded processor is described. Each sensor is connected to a ring oscillator, which translates capacitive changes into a variable-frequency pulse train. An in-depth analysis of the oscillator behavior including all parasitic elements has been performed, and the use of a Schmitt trigger at the inverter chain is proposed to assure oscillation for capacitance values ranging from 1 to 1500 pF. The oscillation frequency is estimated by using a 16-b counter and the appropriate user-defined counting interval ranging from a few microseconds to several seconds in order to achieve maximum accuracy within the entire capacitance range. The 2-B word thus produced can be sent to a host computer through a serial or parallel interface for further processing. To demonstrate the concept, a 16-channel interface has been described in a hardware description language and implemented on a complex programmable logic device. Measurement of two different capacitive pressure sensors developed in-house reveals frequency sensitivity values of -252.1 and -9.54 Hz/fF, which are in good agreement with the expected values derived by the analytic relations.

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Instrumentation and Measurement, IEEE Transactions on  (Volume:60 ,  Issue: 9 )