Inductance - Low Frequency Artifact

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Low Frequency Artifact Produces Inductive Loop
Although most of the instrumental artifacts occur at higher frequencies, low frequency experiments are not immune! Here is one example.

Fleig, et. al. show another instrumental artifact that may appear under some circumstances -- in the study of high impedance, low capacitance systems. The authors discovered this effect looking at the conductivity and dielectric constant of silver bromide crystals. However, some work with coatings or microelectrodes may also show this artifact!

When working with high impedance samples, it is often advantageous to place the sample inside of a conducting box -- a Faraday Cage -- to minimize the effects of stray pickup from the AC line, nearby computers or CRT's, etc. However, there is some small capacitance between the electrodes or wires and the surrounding 'cage' or conducting box. Since both the cage and the electronic measuring equipment are grounded together, some small stray current will flow through the sample then through this capacitance to ground. In a great many applications this current is negligible. However, when the impedance of the sample is high, the stray current may be a significant part of the total current. In the examples reported by Fleig, the impedances were on the order of 107 - 108 ohms and the sample capacitance was only a few pF.

Measurement system sketch

Red:sample; Pink: stray capacitance;
V: voltmeter; A: ammeter;
~ : Signal generator.

The sketch on the left illustrates that, due to current flow through the stray capacitance, the current flowing into the ammeter is smaller than the current which enters the sample! This causes the measured impedance ( = E / I ) to appear to be too big. Although the sample is modeled in the figure as just two RC networks, a distributed network or transmission-line model is a better representation. Fleig showed that the transmission-line model reproduced his AgBr crystal data very well over the entire frequency range, and that the low frequency inductive loop was due to the stray capacitance artifact.


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REFERENCES
"Inductive Loops in Impedance Spectroscopy Caused by Electrical Shielding", J Fleig, J Jamnik, J Maier, J Ludvig, J. Electrochem. Soc., 143 (1996), 3636.

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