Jaan Ojarand

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.

Multisine and Binary Multifrequency Waveforms in Impedance Spectrum Measurement - A Comparative Study

The multisine and binary waveforms are usable as multifrequency excitation signals in measurement of impedance spectrum of biological objects. A comparative study of these waveforms is given for the case of multifrequency signals covering 3 frequency decades with 11 spectral components with octave based frequency distribution. It is shown in the paper that the multisine excitation signal can be optimized by adjustment of phases of the desired spectral components. Moreover, it is concluded that the binary multifrequency signal formed from the corresponding multisine waveform as the signum function of it, is much more effective than the optimized multisine signal.

Shortened square wave waveforms in synchronous signal processing

Measurement of impedance is by definition conducted with sinusoidal waveforms. In many practical cases sinusoidal signals are replaced with suitable approximations. It could be square wave, piecewise continues approximation of the sinusoid or other suitably chosen waveform. In all cases care must be taken to avoid errors introduced by higher harmonics. Square wave signals are easy to generate and energy efficiency is high. Their resulting spectra however are most unfavorable. Piecewise continues approximations of the sinusoid are harder to generate and are sensitive to level errors. Well-known alternative is the three level shortened square wave. While it improves measurement accuracy compared to simple square wave, spectral content leaves room for improvement. By introducing more equally spaced levels, higher harmonics can be reduced. This multilevel signal is still easy to generate digitally and enables simple digital processing involving only additions and shifting.

Crest factor optimization of the multisine waveform for bioimpedance spectroscopy.

The multisine excitation is widely used in impedance measurements to retain the advantages of the sine wave, while reducing the measurement time. Adding up sine waves increases the amplitude of the excitation signal, but, for the linearity assumption to be valid, the overall amplitude of the signal needs to be kept low. Thus, the crest factor (CF) of the excitation signal must be minimized. A novel empirical method for the minimization of the CF is described in this paper. As in the case of other known methods, the computed CF may be guaranteed to be only a local minimum. However, a systematic variation of initial parameters, which is possible due to the sparing algorithm, ensures a CF value very close or equal to the global minimum. The results of CF minimization and comparison with the results from other sources are provided. The direct CF optimization results (set of optimal phases) are not well suited for practical implementation. The influence of phase accuracy on the CF is discussed, and an algorithm for the recalculation of initial phases to the rougher set is described. It is shown that previously obtained optimization results (minimal CF) can be highly preserved, even in the case of rough phase resolutions. The CF of the multisine also depends on the frequency distribution and amplitudes of its components. The CF of multisines with several frequency distributions are compared.

Short-time chirp excitations for using in wideband characterization of objects: An overview

In this paper, certain aspects of choosing excitation waveforms for fast identification of the complex electrical impedance over a wide frequency range are discussed. For this purpose, several chirp-like short-time excitation signals with near to minimal duration are proposed. The results of computer simulation and analysis are promising for implementing such kind of signals as the stimulating ones in the bioimpedance measurement.

Comparison of spectrally sparse excitation signals for fast bioimpedance spectroscopy: In the context of cytometry

This paper presents a comparative study of multifrequency excitation signals that are suitable for fast bioimpedance spectroscopy. If properties of an object are changing fast or if the objects are moving fast, e.g. cells flowing within microfluidic channels, the excitation signal should cover the frequency range of interest in short timeframes. However, since shorter signals carry less energy this reduces the signal-to-noise ratio performance. To enhance performance, it is essential to concentrate the signals energy right on the specific frequencies of interest. Taking into account properties of the biological matter allows us to use more efficient spectrally sparse excitation signals. We also demonstrate that properly designed binary multifrequency excitation signals offer better performance in comparison to multisinewave signals.

Time-frequency analysis of biological matter using short-time chirp excitation

In this paper, using of short-time chirp excitation for the bio-impedance measurement is discussed. The short-time chirp waveforms have at least two advantages. First, flat and wideband amplitude spectrum together with shortness of signal enable fast and adequate estimation of the object under study. Second, low power consumption with more than 90% concentration of the generated excitation energy into the desired bandwidth make such kind of excitation signals especially suitable for applying in implantable and wearable devices of impedance spectroscopy. The appropriate measurement system incorporates matched filtering of the response signal with following spectral analysis to locate small deviation of the actual bio-impedance spectrum from the reference spectrum.

Front-end electronics for impedimetric microfluidic devices

Impedance spectroscopy is a common approach in assessing passive electrical properties of biological matter, however, serious problems appear in microfluidic devices in connection with distortion free signal acquisition from microelectrodes. The quality of impedance measurements highly depends on the presence of stray capacitances, signal distortions, and accompanying noises. Measurement deficiencies may be minimized with optimized electronics and sensing electrodes. The quality can further be improved with appropriate selection of measuring signals and also with selection of measuring methods such as a choice between current or voltage sources and between differential or singleended techniques. The microfluidic device that we present here incorporates an impedance sensor, which consists of an array of two sequential pairs of parallel microelectrodes, embedded in a microfluidic channel. All electronics and fluidic components are placed inside a metal holder, which ensures electric and fluidic connections to peripheral instruments. This configuration provides short electric connections and proper shielding. The method that we are using to evaluate the samples impedance is the differential measurement technique, capable of suppressing the common mode signals and interferences, appearing in the signal-conditioning front-end circuit. Besides, it opens the possibility for compensating stray effects of the electrodes. For excitation we employ wideband signals, such as chirps or multifreqyency signals, which allow fast measurements, essential in the most impedimetric experiments in biology. The impedance spectra cover the frequency range between 10kHz - 10MHz. This is essential for accessing information relating to β-dispersion, which characterizes the cells structural properties. We present two measurement schemes: (i) an in-phase differential method, which employs two transimpedance amplifiers, and (ii) an anti-phase method, which uses one transimpedance amplifier. In this study we analyze and compare the sensitivity, signal-to-noise-ratio, and operational bandwidths of these two methods against other commonly used related circuits.

Optimisation of multisine waveform for bio-impedance spectroscopy

The multisine excitation is widely used in impedance measurements to retain the advantages of the sine wave, while reducing the measurement time. Adding up sine waves increases the amplitude of the excitation signal, but, for the linearity assumption to be valid, the overall amplitude of the signal needs to be kept low. Thus, the crest factor (CF) of the excitation signal must be minimized. A novel empirical method for minimization of the CF is presented in this paper. As in case of other known methods, the computed CF may be guaranteed to be only a local minimum. However, a systematic variation of initial parameters, which is possible due to the sparing algorithm, ensures the CF value, very close or equal to the global minimum. A brief analysis of the calculation errors and comparison with the results from other sources is provided. In the DSP applications, initial phases of the waveforms must match the grid of sample points if separate signal sources are used to generate the components. An equation for calculation of the indexes of the sample points corresponding to the optimal initial phases is provided, and the time discreteness caused relative error of CF is illustrated.

Optimization of multisine excitation for a bioimpedance measurement device

Multisine excitation is widely used in impedance measurements to retain the advantages of the sine wave, while reducing the measurement time. To keep the crest factor (CF) of the excitation signal low, the initial phases of the signal components must be optimized. This paper focuses in further optimization of multisine signal for improving the signal-to-noise ratio (SNR) of measurements, reducing the complexity of signal generation and minimizing a memory footprint of the FPGA based implementation.