Fitting Electrical Impedance Spectra Measured by an MFIA Impedance Analyzer

August 21, 2023 by Meng Li

Equivalent circuit modeling (ECM) is often necessary to understand the detailed frequency-dependent physics behind a device or component. In many real-world scenarios where the circuit model is complicated (more than 2 elements), ECM can only be done after a frequency sweep is completed, or in other words, during post-processing. In this blog post, we will present an easy way to perform ECM in a third-party program, developed by Prof. Nita Dragoe from Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO) at Université Paris-Saclay.

Figure 1. LabOne Sweeper screenshot showing the Nyquist plot of an LED DUT at 3 different forward biases. Green: 1.375 V; Blue: 1.4 V, Red: 1.425V.

Our device-under-test (DUT) is a red LED mounted to an MFIA Impedance Analyzer via an MFITF fixture. In Figure 1, we measure the electrical impedance spectra (EIS) between 1 kHz and 5 MHz at 3 different forward DC biases: 1.375 V (in green), 1.4 V (in blue), and 1.425 V (in red), respectively. Note that the equivalent circuit shown in LabOne is designed for fast real-time reading at a fixed frequency and only supports 2 circuit elements (such as Rp||Cp). When forcing this oversimplified model on our DUT represented by 3 elements (Rs+Rp||Cp), the raw impedance (Z and phase) stays unaffected, but the derived resistance and capacitance can be unreliable. Therefore, to properly fit the EIS, we export the data from LabOne into .csv file format as an example, but it is worth noting that .txt format is also supported.

Figure 2. Screenshot of a third-party program showing the fitting of the EIS data taken by an MFIA.

The .csv files saved by LabOne can be loaded directly into the third-party program in a few mouse clicks without additional parsing and restructuring of data. What's more, we can also import multiple traces together, and selectively display any of them on the screen. In Figure 2, the 3 traces are shown in the same color coding as in Figure 1, by scattered points. To enable the fit in solid lines, we can go to the 'Model' tab and add Rs+Rp||Cp model there in a graphical way, shown in Figure 3. Finally, by clicking the 'Fit selected' button, we obtain 3 nice fits all at once. The fitted results can be further found under the 'Action' button.

Figure 3. Advanced models such as Rs+Rp||Cp can be graphically built in the 'Model' tab of the software.

In this blog post, we have explained how to quickly load and fit the EIS data taken by the MFIA, using a third-party program. We hope this program can help to simplify your workflow of impedance applications.

Reference

We would like to thank Prof. Nita Dragoe from Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO) at Université Paris-Saclay for developing "Yet Another Program for Analysis and Research in Impedance". The program can be referenced in publication as http://dx.doi.org/10.13140/RG.2.2.15160.83200