Time-Frequency Design of a Multi-Sine Excitation With Random Phase and Controllable Amplitude for (Bio) Impedance Measurements

Abstract

Impedance spectroscopy has become a standard electroanalytical technique to study (bio)electrochemical and physiological systems. From an instrumentation point of view, the measurement of impedance can be carried out either in the frequency domain using the classical frequency sweep method or in the time domain using a variety of broadband signals. While time-domain techniques can be implemented with relatively simple hardware and can achieve faster acquisition time, they are still not that popular because of their lower accuracy and modularity. In this work we present a method and an algorithm for computing the impedance spectrum of samples using an arbitrary time-domain signal excitation initially engineered from a multi-sine summation. The signal is designed in such a way that its spectral phase is derived from a discrete uniform distribution (obtained from a physically-true random number generator), but its time-domain maximum amplitude remains controllable. As such, samples can be excited with adjustable amplitude signals (as low as 50mV to 2V in the current setup) in order to avoid exciting the nonlinearities in the samples under test, which is critical for the very definition of the impedance. Furthermore, the method is proved to combine both speed of measurement, given that it permits to excite at once a large number of frequency components of the sample, as well as accuracy given that the signal’s power spectrum is relatively flat. We verified our method and algorithm for monitoring yogurt quality using (bio)impedance measurements over a period of 48 hours.

Keywords

• (Bio) impedance spectroscopy
• signal processing
• random signal
• biosensor