FPGA‐Based Processor for Continual Capacitive‐Coupling Impedance Spectroscopy and Circuit Parameter Estimation

Abstract

In principle, the recently proposed capacitive‐coupling impedance spectroscopy (CIS) has the capability to acquire frequency spectra of complex electrical impedance sequentially on a millisecond timescale. Even when the measured object with time‐varying unknown resistance Rx is capacitively coupled with the measurement electrodes with time‐varying unknown capacitance Cx, CIS can be measured. As a proof of concept, this study aimed to develop a prototype that implemented the novel algorithm of CIS and circuit parameter estimation to verify whether the frequency spectra and circuit parameters could be obtained in milliseconds and whether time‐varying impedance could be measured. This study proposes a dedicated processor that was implemented as fieldprogrammable gate arrays to perform CIS, estimate Rx and Cx, and their digital‐to‐analog conversions at a certain time, and to repeat them continually. The proposed processor executed the entire sequence in the order of milliseconds. Combined with a front‐end nonsinusoidal oscillator and interfacing circuits, the processor estimated the fixed Rx and fixed Cx with reasonable accuracy. Additionally, the combined system with the processor succeeded in detecting a quick optical response in the resistance of the cadmium sulfide (CdS) photocell connected in series with a capacitor, and in reading out their resistance and capacitance independently as voltages in real‐time.