PEDRA uses a different approach to fitting EIS data.

1) Instead of providing a equivalent circuit model (ECM), PEDRA uses an Expandable Randles (cascading resistance) or Debye (parallel resistance) ECM.

2) The number of responses in the model is dependent on what the data can support.

3) Initial parameters are obtained directly from a plot of the data.

4) The type of response is identified by the parameters that describe it; either circuit (Resistance, Capacitance, power law term) or Physical (Resistance, dielectric thickness, fractal or factional distribution).

5) Displaying results using a 3-parameter Gaussian equation allows results to be presented as an overlay graph directly within PEDRA.

PEDRA supports three methods of importing data: 1) legacy files, e.g. PARC, Solartron, Gamry, 2) delimiated files having using an interface to delete header information and choose data columns of frequency, real and imaginary impedance, 3) copy and paste spreadsheet from a spreadsheet.

1) Import multiple data files into the experiment.

2) Estimate how many responses maybe in the spectra (this can be changed during the fit).

3) Set the parameter constraints (optional) using the Preferences menu. This helps in fit convergence.

4) Perform autofit. The process will return the last successful fit iteration.

5) Continue with user iterative fit process until convergence is achieved (i.e. hold and release parameters observing fit, fit residuals and parameter error).

6) Continue fitting data files. Any previously fitted data on the list can be overlayed on current file for comparison.

7) Once data files are fit, display results on an overlay plot with either the frequency (time constant) or resistance as the x-axis.

8) Use the extensive capability of IGOR PRO to create publication quality graphs.

9) Export fit results in spreadsheet form that includes experiment information, fit results and columar data/fit columns.

First, everything is on one screen using three panels.

1) The Main Graph panel contains a ribbon menu and a graph of fit results with options to choose axis, a) Ratio (Zimag/Zreal), SpecView (Gaussian line graph), b) Bode plot, either Zreal, Zimag or Zmag, Phase Angle.

2) The Information Panel contains all fitting controls and fit results (parameters, parameter error, response influence and time constant).

3) The Fit Control Panel contains all imported files each with fit and overlay check boxes.

• Currently PEDRA 9.0 works primarily for high impedance barrier films. Extending its use for low impedance application will require additional develoment.
  • Proposed Fix: a suitable IGOR PRO programer will have to modify the open source program to include low impedance data the may include inductance or stray capacitance.
• SpecView Capacitance: When choosing Circuit Parameters (CIRC) the y-axis of the SpecView Plot is labeled Capacitance, however the peak heights are held at an abitrary height of '10' and only indicate peak positiion and width. (Values of the capacitance could be added as text when used in reports.)
  • Proposed Fix: When using CIRC parameters the y-axis will be a reverse axis of capacitance. It will show values of capacitance in reverse (linear values on log axis from high to low). Thus, peaks will be similar to thickness but in units of capacitance.
• SpecView Base Line: In Preferences the SpecView Base Line has a default value of 0.01. The axis on the SpecView Plot will have a minimum of 0.01 and maximum 'nice' value, e.g. maximum 800 will become 1000.
  • Proposed Fix: The Baseline value determines the number of decades show by the axis. e.g. 3 would show a three decade range below the peak height, either max value of thickness or min value of capacitance (capacitance is plotted on a reverse axis.
• Boundary Responses - currently PEDRA 9.0 does not handle boundary responses well.
  • Seriers Inductance - The series resistance is reduntance to the high frequency resistance parameter and needs to be held << R1. The fitting algorythm does not well for negative imaginary impedance.
    • Proposed Fix: Remove the series inductance resistance parameter and improve fit capability
  • Series Capacitance - Implimented incorrectly; should be in parallel with EECM rather than in series.
    • Correct program
  • Low frequency inductance - has not been implimented yet.
    • The series resister inductance term needs to be in parallel with the low frequency impedance term.