Technique of increasing the impedance measuring transducers accuracy at inharmoniousness signals

Abstract

Introduction. Nowadays, one of the most relevant classes of sensor electronics for the Internet of Things is based on the methods of impedance spectroscopy. In general, the informative signals of impedance spectroscopy, namely, an active component or resistance which forms the real part and a reactive component or reactance which forms the imaginary part of complex impedance of the investigated two-terminal network, are formed by specialized analog front-end or measuring transducers. The defining requirements for measuring converters of sensor devices of the Internet Things are their versatility, minimal power consumption, the ability to work at low supply voltages, stability of operation with changing external factors and so on. In this paper, the problem of improving the accuracy of electrical impedance measuring transducers is considered, taking into account the non-harmonic of the driving signals.
SPICE model and research technique. The implementation of impedance spectroscopy assumes a transition from frequency plots to plots on the complex plane, called as Nyquist plots. In a number of modern versions of circuit simulation programs the method of impedance analysis is already provided with the use of mathematical functions of Real (Re) and Imagine (Im) components of the signal. Using these functions, it is possible to calculate the corresponding values of the active (Re Z) and the reactive (Im Z) impedance of the object under investigation. As a result of his usage a Nyquist plot is plotted. However, this approach is not universal, and in particular, imposes significant restrictions taking into account the parameters of real signals, namely, their amplitude, shape, non-harmonics, etc. The results obtained in this paper are based on the new SPICE (Simulation Program with Integrated Circuit Emphasis) model studding methodology, which compares small signal Alternative Current Analysis with large signal Transient Analysis. During the Alternative Current Analysis, Nyquist impedance plot are obtained in the idealized case, and during the Transient Analysis the active Re Z value and reactive Im Z impedance components are calculated for the actual parameters of the measuring transducers and the form of the activating signals. The implementation of the above mentioned methodology involves the use of synchronous quadrature detection of the output signals of the measuring circuit.
Analysis and correction of results. In accordance with the task of increasing the impedance measuring transducers accuracy at inharmoniousness signals we consider the method of calculating the coefficients KRE and KIM, allowing correcting the measurement results of the active Re Z and the reactive Im Z impedance components. A few analysis and correction results are presented. The obtained regularities can be widely used in correcting the results of impedance spectroscopy upon activation by pulse signals for the vast majority of research options.
Conclusion. A new approach is proposed, according to which three arrays of informative signals of active Re Z and reactive Im Z impedance are formed and compared. The first M(AC) array is obtained using small signal Alternative Current Analysis, which corresponds to an idealized version of the measurement transformation and is subsequently used as a reference. The next two arrays M(H1) and M(HN) are obtained by the method of large signal Transient Analysis using the results of integrating the quadrature detector output voltages of the impedance transducer. The array M(H1) is formed when the input source triggers a harmonic oscillation (the first H1 harmonic), and the M(HN) array when activated by a functionally controlled source synthesizes a non-harmonic signal in the form of harmonics HN. The increase in the accuracy of the measurement conversion is provided by coefficients $K_{RE}$ and $K_{IM}$, which allow the correction of the measurement results of the active and reactive components of the impedance. A method for calculating such coefficients and examples of their use are presented.