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invfreqs and Warburg impedance

From: Pascal A. Dupuis
Subject: invfreqs and Warburg impedance
Date: Mon, 20 Dec 2004 09:22:57 -0600
User-agent: Mutt/1.5.6+20040907i


I'm trying to describe an Impedance measured over a few frequencies
points as a combination of resistors/capacitors, using the invfreqs
function. But the underlying system is comprised of a pair of
electrodes, and some biological medium, so the impedance contains a
so-called Warburg element, that is, a capacitor in parallel with a
resistor, where the phase angle is constant (45 °) and the product of
R and C is equal to 1/f.

When I plot the impedance locus (real part, imaginary part), it is
obvious that there is something going wrong with the approximation
using constant R and C. Did someone already coped with this, coming to
some iterative method permitting to extract the Warburg element and
progressively come to a good approximation of the measured impedance.


Pascal Dupuis

Dr. ir. Pascal Dupuis
K. U. Leuven, ESAT/ELECTA (formerly ELEN):
Kasteelpark Arenberg, 10; B-3001 Leuven-Heverlee, Belgium
Tel. +32-16-32 10 21 -- Fax +32-16-32 19 85

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