Marko Reidla

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.

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.

Capacitance measurement with MSP430 microcontrollers

Capacitive sensors are widely used, e.g. for measurement of humidity and various physical movement (proximity, linear or angular acceleration, speed or position) of some element in the environment or in the material or structure. MSP430 micro-controllers are often used in wireless sensor node applications, as being low-power low-cost mixed signal systems on chip with various (including wireless) connectivity options. So, among other applications, the development of MSP430-based sensor nodes for smart materials and structures could be highly interesting, with capability of capacitive sensor interfaces. MSP430-s have been much used with capacitive touchpad inputs, but less investigated for the use from the instrumentation viewpoint. In the current paper two implementation ways of the capacitive measurement were investigated, from practical and from the metrological aspects, one implementation as the pin-relaxation-oscillator (possible with MSP430G2 family) and another solution, implemented as the internal comparator-based relaxation-oscillator solution (with MSP430F5 family). The results indicate that MSP430 micro-controllers can be easily (by simple interface) connected to the capacitive sensors for reasonably precise measurements (measurement error is below ± 1 pF in the range of 10 to 600 pF. One of the future research challenges could also be the implementation of the sensor nodes with conductive fibre or layer (with changing resistance and capacitance to be measured) for structural health monitoring.

TMS320F28335-based high-accuracy complex network analyzer instrument

Preliminary prototype of the TMS320F28335- (digital signal controller-DSC)-based complex network analyzer instrument has been developed and investigated. High accuracy (16-18 bit resolution) and relatively wide analogue bandwidth (up to 10 MHz) is achieved by using of external ADC and DAC devices. Using of DSCs offer much computational power for real-time signal processing (efficient Dual MAC operation), dedicated memory and IO controllers (DMA) and flexible communication and peripheral interfaces at low cost and reasonable power consumption. Proposed solution has been developed, described and discussed. Performance estimations, both from theoretical considerations, as well as from experimental investigations, has been given. As a result, a a very promising technical solution has been developed. Main contribution of the current work has been practical implementation and investigation of specific (high-accuracy) analog interfaces and corresponding firmware parts.

Feasibility study: A DM3730-based data acquisition and processing solution

DM3730 (and OMAP3530) high-performance, multicore processor based development boards (such as the Beagleboard-xm) with according software solutions and development tools is an efficient platform for various computing, signal and image processing applications, with various connectivity options. For the high performance impedance (and complex network) spectrum analyzer solutions, a critical role play realization of the data converters (ADC and DAC) and their interfacing to the processor. Analog-to digital conversion can be performed by external 16-18 bit ADCs, with serial interfaces working in the under-sampled mode. For the DAC part the main challenge is the generation of the accurate multi-frequency (better, if arbitrary) excitation waveforms. Using of the processors internal 10-bit video DACs (having many benefits, compared with using of the similar external DACs) for this purpose have been investigated, for the possibly high accuracy (14-18 bit effective overall accuracy) and wideband (up to 5 to 10 MHz, analogue signals) arbitrary waveform generations. First experiments show good hope to develop an efficient and cost- and form- effective OMAP (open multimedia application platform) based impedance (or complex network analyzer) solution with very competitive metrological characteristics. Investigation results so far are described.