ELECTRICAL MEASURMENT
order to check impedance measuring equipment you again need standard resistors as a minimum. Other parameters may also affect the accuracy; linearity and, not least, temperature stability. Most measuring equipment specify the limits of error as a function of temperature or deviations in temperature. Keep this in mind; note the ambient temperature in your log-book, check that the fan of the equipment works, and that the fan and instrument interior is not clogged by dust.
Parasitic impedances and admittances When measuring impedances we have to be aware of things that add to the impedance, i.e. things that act in series with our device or sample under test, but are not intended to be part of the result. These are called parasitic impedances. We also have to be aware of things that are adding to the admittance of the measurement – things that are in parallel and thus contribute parasitic admittances. The figure below illustrates the action of parasitic impedances and admittances. One may object that the connection points of the two with respect to each other is not obvious, but in practice only one makes a significant contribution at a time, and then the order of connection is not an important issue.
Figure 13. Parasitic impedance Zp in series and admittance Yp in parallel with a device D.
Parasitic impedances comprise the resistance and inductance in the wires that lead to the sample. The resistance is typically of the order of an ohm or less. They can be eliminated by 4-wire measurements. Next we have spreading resistance in the electrode, contact resistance to the material or electrochemical resistance in the case of an ionic conductor. If these are not part of the measurement of interest, they can be eliminated by using 3- or 4-electrode measurements. Finally, resistance and inductance remain as two elements that make up the impedance of the sample or component under test.
Figure 14. Parasitic impedance elements; resistance and inductance.
Parasitic admittances are parallel to the sample and thus comprise all possibilities that current has to flow between the two sides of the sample. In DC measurements this includes transport on and in insulators. In particular, adsorbed humidity on surfaces provides some conduction. In AC measurements, signals may furthermore be transmitted between wires across the capacitance in air or insulators. These sources of parasitic admittances are usually attempted eliminated by shielding the conductors from each other (see below). Parasitic admittance furthermore includes transport on the surface of the FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +91-999954621
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