Measuring impedance passive circuit

The technique presented here can be used to measure the impedance or resistance between any two nodes. We will find the AC impedance between two nodes. We will illustrate using two examples. The first will be a passive circuit with resistors only. The second will be a jFET source follower. 

Figure 19 Schematic Bode plots from EIS measurements and equivalent circuits that could be used to fit them for various possible corrosion product deposit structures (A) nonporous deposit (passive film) (B) deposit with minor narrow faults such as grain boundaries or minor fractures (C) deposit with discrete narrow pores (D) deposit with discrete pores wide enough to support a diffusive response (to the a.c. perturbation) within the deposit (E) deposit with partial pore blockage by a hydrated deposit (1) oxide capacitance (2) oxide resistance (3) bulk solution resistance (4) interfacial capacitance (5) polarization resistance (6) pore resistance (7) Warburg impedance (8) capacitance of a hydrated deposit.

V0/V/ (fti), to the analytical expression with recovery of the complete quartz impedance near resonance (admittance, conductance and impedance). Although the voltage divider method does not measure the transfer function phase and hence it is not possible to demonstrate the validity of BVD circuit, it has the advantage of speed. Also passive methods like TFM can be applied under high viscous damping so that the shear wave phase never crosses zero and the EQCM no longer resonates. 

Immittance — In alternating current (AC) measurements, the term immittance denotes the electric -> impedance and/or the electric admittance of any network of passive and active elements such as the resistors, capacitors, inductors, constant phase elements, transistors, etc. In electrochemical impedance spectroscopy, which utilizes equivalent electrical circuits to simulate the frequency dependence of a given elec-trodic process or electrical double-layer charging, the immittance analysis is applied. 

EIS measurements can also be carried out under conditions where illumination of the semiconductor generates a photocurrent. The technique is then referred to as photoelectrochemical impedance spectroscopy, PEIS. Interpretation of the results in terms of passive RC circuit elements is no longer appropriate since the system contains a current source. A more satisfactory approach is to relate the impedance response directly to the physical processes responsible for the photocurrent (Ponomarev and Peter, 1995 Peter, 1999 Peter and Vanmaekelbergh, 1999). 

In homogeneous corrosion systems (active dissolution, passive state) where the same electrochemical reactions occur over the whole surface, the interrupter and ac technique can be successfully applied and the same value for the ohmic resistance is measured by both techniques. Problems arise in localized corrosion systems, where small active areas coexist with a large passive surface and the impedance of the active areas (pits) is short circuited by the surrounding passive surface. 

Usually the impedance is measured as a ftmction of the frequency, and its variation is characteristic of the electrical circuit (where the circuit consists of passive and active circuit elements). An electrochemical cell can be described by an equivalent circuit. Under appropriate conditions, i.e., at well-selected cell geometry, working and auxihary electrodes, etc., the impedance response will be related to the properties of the working electrode and the solution (ohmic) resistance. 

However, it is important to note that the Ret values are not directly related to the susceptibility to corrosion of the different alloys and composites. They are related to the rate of charge transfer reactions that give rise to the formation of a passive layer on the surface of the samples (the impedance measurements are at open circuit potential only). The protective characteristics of these passive films depend on the preparation conditions of the alloys, the distribution of elements in the alloy and the presence on the surface of active sites for adsorption of chloride ion. 

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