Digital and build-up techniques for electrical impedance metrology

This dissertation presents my activity on electrical impedance metrology carried out during my PhD. The dissertation is structured in five main chapters. After a short Introduction briefly describing the research activities, the three central chapters are as follows. My work on the development of digital impedance bridges is described in chapter 2, where I present the implementation of two different digital bridges and some test measurements. The first bridge implemented is a fully digital impedance bridge with a very simple architecture, which is based on a commercial two-channel digital signal synthesizer and a synchronous detector. The bridge can perform comparisons between the impedances having arbitrary phase and magnitude ratio. The bridge balance is achieved automatically. R-C and C-C comparisons with calibrated standards, at kilohertz frequencies and 100 kΩ magnitude level performed to give ratio errors of the order of 10−6 to 10−5. The second implementation is that of a three-arm current comparator impedance bridge which allows one to perform comparisons among three unlike impedances. The main aim of this digital bridge is the calibration of impedances having arbitrary phase angles against calibrated pure impedances. The analyses of the bridge setting and of its operation are presented. To test the bridge, the measurements of an air-core inductor and of an RC network versus decadic resistance and capacitance standards, at kilohertz frequency, are performed. The bridge measurements are compatible with previous knowledge of the standard values with relative deviations of parts in 10−5. In chapter 3, I describe a capacitance build-up technique for the calibration of the nonlinearity of a capacitance meter. The method is implemented with a new design matrix formulation. Two different applications of this method are vii also described. In the first case, the method is applied to the determination of the nonlinearity of an Agilent mod. E4980A precision LCR meter in the range from 1 nF to 10 nF. The results obtained in this case are then compared with measurements performed with a high-accuracy capacitance meter, an Andeen-Hagerling mod. 2500A, whose readings give the reference values. The second experiment is that of a capacitance transfer from 1 nF to 100 pF as a step in the realization of a capacitance scale. The capacitance ratio C(1nF)/C(100pF) obtained with the capacitance build-up method is then compared with the ratio obtained from the Italian national capacitance scale. The uncertainty of the capacitance build-up technique obtained at the 10−6 level. After two chapters dedicated to primary metrology, chapter 4 concentrates on the application of impedance measurements to neuroscience. The shrinking size of microelectrodes leads to high electrode-tissue interfacial impedance and inaccurate measurement when recording the data from the tissue. In order to achieve safe and efficient electrode performances, a porous and rough electrodeposition material to modify the bare microelectrodes is proposed. Combining the advantages of platinum (Pt) with iridium (Ir), gives a microelectrode with low resistance and high capacitance. These new microelectrodes were tested in two different applications. In the first one, fiber carbon microelectrodes with electrodeposited Pt-Ir were fabricated and implanted in the tissue of a rat to record brain signals. In the second application, Pt-Ir nano-wires were electrodeposited on a polymide test substrate. The fabrication process is explained and the optical and SEM images were taken to analyze the roughness of the microelectrode surface. Furthermore, it demonstrated superior mechanical and electrochemical stability of these coating nano-wires on the microelectrode. Morphological tests showed that Pt provided large area and hence good adhesion for dense Ir deposition, which was beneficial for long-term mechanical stability of the composite coating. Finally, the conclusion sums up the results and outlines possible future developments.