Capacitive-Coupling Impedance Spectroscopy Using a Non-Sinusoidal Oscillator and Discrete-Time Fourier Transform: An Introductory Study

Abstract

In this study, we propose a new short-time impedance spectroscopy method with the following three features: (1) A frequency spectrum of complex impedance for the measured object can be obtained even when the measuring electrodes are capacitively coupled with the object and the precise capacitance of the coupling is unknown; (2) the spectrum can be obtained from only one cycle of the non-sinusoidal oscillation waveform without sweeping the oscillation frequency; and (3) a front-end measuring circuit can be built, simply and cheaply, without the need for a digital-to-analog (D-A) converter to synthesize elaborate waveforms comprising multiple frequencies. We built the measurement circuit using the proposed method and then measured the complex impedance spectra of 18 resistive elements connected in series with one of three respective capacitive couplings. With this method, each element’s resistance and each coupling’s capacitance were estimated independently and compared with their nominal values. When the coupling capacitance was set to 10 nF or 1.0 nF, estimated errors for the resistive elements in the range of 2.0–10.0 kΩ were less than 5%.

Short-Time Impedance Spectroscopy Using a Mode-Switching Nonsinusoidal Oscillator: Applicability to Biological Tissues and Continuous Measurement

“…If . Z X is a resistive element R X and capacitive coupling C SX in series, and parasitic capacitance C A can be ignored, the oscillation frequency f 0 can be expressed with the following [24]:…”

“…The procedure for estimating impedance is determined using a previously reported procedure [24]. The voltages v 1 (t) and v 12 (t) measured in the circuit, shown in Figure 1, form the periodic nonsinusoidal waveforms of the oscillation frequency f 0 .…”

“…Therefore, noninvasive impedance spectroscopy targeting insulated resistors, for instance, a living body covered with insulator-like clothing, has rarely been performed. To address this gap, we previously proposed capacitive coupling impedance spectroscopy (CIS) as a noninvasive technique for measuring impedance spectra through an insulation coating [24]. In this method, the measured object and capacitive coupling part are incorporated into an oscillation circuit.…”

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

“…In our previous studies [32,33], it was confirmed offline in [32,33] that the time-varying impedance can be obtained in milliseconds using the novel algorithm of the CIS approach. From this result, it was clarified that the CIS approach can obtain EIS on the order of milliseconds in principle.…”

“…Recently, we proposed capacitive-coupling impedance spectroscopy (CIS) [32] as a novel, noninvasive EIS method for an insulated resistor. CIS is known to have the capability to acquire frequency spectra of complex electrical impedance in a sequential manner on the millisecond timescale, even when the measured object with time-varying unknown resistance R x (t) is capacitively coupled with the measuring electrodes with time-varying unknown capacitance C x (t).…”