PSI - Issue 43

Dagmar Faktorová et al. / Procedia Structural Integrity 43 (2023) 288–293 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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and thin layers according to their physical properties (conductivity, thickness, roughness – mainly with the use of eddy currents), see Joshi (2017), changes in the material properties due to electrochemical processes, typically corrosion and oxidation, determination of properties of dielectric materials, electrolytes, composite and polycrystalline materials and determination of dielectric relaxation and porosity of materials. The basic of the method is the measurement of the complex electrical impedance of the material sample as a function of the frequency of the applied electrical voltage or current. Typical values of applied voltages are in the range of tens of millivolts to the units of volts and frequencies in the range of units of Hz up to tens of MHz. The subject of the paper is a description of the possibilities of an impedance converter implementation for low-frequency impedance spectroscopy in the frequency range up to 100 kHz in a compact miniaturized integrated form which is suitable for in-situ measurements and which is implemented on the STM32 microcontroller platform. 2 Impedance converter concept – circuit description Impedance is generally a complex quantity that can be nonlinear and non-stationary, depending on the nature of the material being measured. At present, for the measurement of the complex impedances there is an extensive background of equipment from specialized devices to configurations using universal devices such as lock-in amplifiers, see Graaf and Wolffenbuttel (2012). An overview of the electronic circuits of the impedance converter devices and their properties are given e.g. in Agilent, Inc. (2013). One suitable method for impedance measurement for low frequency applications is a method in two-electrode configuration, which uses the material sample excitation from a voltage source and indirect measurement of the current using active current-voltage (C/V) converter, Fig. 1.

Fig. 1. General method of impedance measurement using C/V converter circuit. Z x is the unknown impedance and resistance R ref determines the electric current magnitude.

The advantage of this arrangement is the simple circuitry, the possibility of measuring the impedance at the floating potential, the disadvantage is the smaller dynamic range and the need for calibration to eliminate linear distortion which is caused by parasitic impedances of used components. Block diagram of a typical impedance converter in a two-electrode arrangement is shown in Fig. 2. DDS is a tunable harmonic generator usually implemented by direct digital synthesis technology, DAC is a digital-to-analog converter with the possibility to change the output voltage range, PGA is programmable gain amplifier for adjusting the signal dynamics and LF is a low-pass bandwidth limiting filter for analog-to-digital ADC converter. CPU is a processor for real and imaginary parts of the complex impedance calculation. A compact impedance analyzer for measuring impedances in the range of 1 kΩ to 10 MΩ for frequencies up to 100 kHz which is based on the mentioned method is available as a separate integrated circuit under the name AD 5933, see Analog Devices, Inc. (2017). The circuit includes a mathematical processor for real and imaginary parts of the complex impedance calculation and the I 2 C interface for controlling the analyzer by a microcontroller.