Marek Rist

Binary signals in impedance spectroscopy

Using of binary waveforms in the fast impedance spectroscopy of biological objects is discussed in the paper. There is shown that the energy of binary waveforms can be concentrated onto selected separate frequencies. We can optimize the binary excitation waveform depending on the shape of frequency response of the impedance under study to maximize the levels of signal components with certain selected frequencies. As a result, we are able to receive maximal amount of information about the properties and behavior of the impedance to be studied. We have designed and prototyped the impedance spectroscopy device operating in the frequency range from 100 mHz to 500 kHz to cover α- and β-regions of the bio-impedance spectrum of time-varying subjects as, for example, fast moving cells in micro-fluidic devices, beating heart and breathing lungs or the whole cardiovascular system.

Improved impedance analyzer with binary excitation signals

The impedance analyzers are widely used to measure, test and validate the properties of the materials, tissues, electrical and electronics networks etc. To make simple power-and cost-efficient device for real-time dynamical measurements in the wide frequency range, special binary waveforms excitation signals can be used, covering the desired spectrum points. Drawback of the using of the binary measurement signals is the presence of the aliased spectrum part in the response signal, giving reasonable measurement error in some cases. In the current paper the solution is proposed, where the measurement error is corrected by the reverse estimation and modeling of the circuit-under-test and the over-all measurement device and system. Examples of the real measurements and improvements are described, for the Piccolo DSP-chip based solution, working in the frequency range from 1 to 500 kHz (15 simultaneous frequencies, with 1 ms output refresh-rate), where the excitation signal is generated by the advanced PWM-unit of the Piccolo-chip and the response signal is acquired by the on-chip 12-bit ADC of the same Piccolo chip and processed on the place. By using of the proposed and described estimation-correction solution the impedance measurement error can be decreased from several tens of ohms till few ohms or even less, if using the fixed 1 kOhm resistor /Zx (measured impedance) voltage divider for the measurement circuit.

Modular System for Spectral Analysis of Time-Variant Impedances

Electrical bioimpedance of living tissues changes over time due to heart beating and breathing. To measure the changes in impedance spectrum requires sophisticated instrumentation. In this paper, a modular system for fast measurement of the time-variant spectrum of the time dependent (dynamic) impedance is presented. The system enables to measure the dynamic impedance simultaneously at up to 16 frequencies from 1 to 400 kHz within 1 ms time slots. Repeating the measurements after predetermined intervals gives the results in the form of spectrograms. Experimental results characterizing the case of dynamic measurement of impedance variations caused by heart beating and breathing are given in the paper.

Phase-increment sampling in chirp signal based impedance measurements

For measuring of the spectra of complex electrical impedance, the chirp excitation signals can be used. The analysis of the response signal of an object, often dynamical, in time, or circuit under test can be performed by short-time Fourier transform in the sliding time-domain window. Computationally more efficient and accurate could be the usage of the simple correlation (multiplication-and accumulation of the discrete samples in the digital processing case) by the excitation waveform (sine chirp) as the first reference signal in the first (“in-phase”) channel and by corresponding cosine-chirp in the other (“quadrature”) channel, both in the same sliding in time window. Still, classical uniform sampling scheme in such solutions is not efficient and not giving accurate result, so the non-uniform sampling scheme of excitation signal and reference waveforms has been proposed and investigated, on some practical examples. For linear chirp - the key is usage of the constant phase increments of the sampling instants of the excitation signal, corresponding to the non-uniform time and frequency increments. Simulations for measurement of the impedance spectra in the frequency range of 10kHz to 1 MHz with 1 ms duration chirp signal has been performed, showing the good performance of the proposed approach at possible simplicity of the implementation.

Comparison of excitation signals and methods for a wideband bioimpedance measurement

Various excitation signals are applicable for the wideband impedance spectroscopy. However, in electrical bioimpedance (EBI) measurements, there are several limitations that set specific demands on the properties of the excitation signals. This paper compares the efficiency of different excitation signals in a graspable presentation and gives recommendations for their use. The paper deals with finding the efficient excitation waveforms for the fast spectroscopy of bioimpedance. Nevertheless, the described solutions could be useful also in other implementations of impedance spectroscopy intended for frequency domain characterization of different objects.

Notes on applicability of the impedance spectroscopy for characterization of materials and substances

Practical aspects of the applied impedance spectroscopy are considered. A concept and demonstrator for embeddable impedance spectroscopy device are discussed. The proposed concept is based on the use of simple binary signals as for excitation. Signal generation as well as later processing of response signals is handled inside single chip digital signal processor. Minimal amount of supporting circuitry is added, and set of exchangeable front end units is added. Low level complexity, low power consumption and low cost of implementation are the criteria besides the strict requirement for the stability and repeatability of the operation. Three different application scenarios are briefly introduced. Achieved results are evaluated in the lines of accuracy versus repeatability, and fulfillment of requirements.

Dynamic reference for evaluation of bioimpedance spectroscopy devices

Developed testing platform is intended for creating dynamic time dependent impedance in order to evaluate the performance of impedance measurement devices. The device is targeted mostly for biomedical impedance devices and other applications where evaluation by using time invariant objects does not give enough information about the adequacy of impedance measurement device for the current application.

Enhanced optimization of the wideband excitation signal for a bioimpedance measurement

Low crest factor (CF) of the excitation signal is commonly considered as an important factor for improving the signal-to-noise ratio (SNR) of bioimpedance measurements. However, the overall SNR of impedance measurements also depends on the spectral properties of the response signal, which, in turn depends on the properties of the sample under test (SUT). This paper focuses in further optimization of wideband excitation signals, taking into account also the properties of SUT and lesser complexity of the measurement setup. The paper deals with finding the highly efficient multifrequency excitation waveforms for fast bioimpedance spectroscopy. However, the solutions described here could be useful in other implementations of impedance spectroscopy and also in frequency domain characterization of different objects.