Fitting

Circuit fitting

Fit EIS data to an equivalent circuit and read off solution resistance, charge-transfer resistance, double-layer capacitance, and diffusion parameters.

Circuit fitting converts an impedance spectrum into physical quantities. The interface is modelled as a small electrical circuit whose elements correspond to physical processes, and the element values are adjusted until the simulated impedance matches the measured spectrum.

Studio has 17 built-in circuits and a custom circuit editor. Auto Fit runs every built-in circuit and ranks the results.

When to use it

Characterising new electrodes: Rct quantifies how easy electron transfer is, Cdl reports on the electrochemically active area.
Coating integrity: a defect-free coating shows a single large semicircle; pinholes and water uptake introduce a second time constant.
Battery and fuel-cell diagnostics: separating ohmic loss (Rs), interfacial loss (Rct), and mass-transport loss (Warburg).
Mechanism discrimination: different mechanisms map onto different circuits, and the fit shows which one the data supports.

How it works

Pick a circuit. Studio estimates starting values from the shape of the Nyquist trace and then runs a non-linear least-squares fit against the complex impedance at every measured frequency.

Fit quality is reported as R² and χ² together with a residual plot. Flat residuals mean the model describes the data; a systematic trend means the circuit is missing an element.

Points can be excluded from the fit by clicking them or dragging a box in the fit window, and the frequency range set in Data Processing is respected.

Circuit fitting in Sensitify Studio

Parameters

Circuit

Typical: Randles + Warburg for aqueous redox

One of the 17 built-in circuits (Randles, Randles with Warburg, two time constants, and their variants) or a custom circuit composed from R, C, CPE, L, and W elements.

Auto Fit

Fits every built-in circuit and ranks them by AIC, with R² and χ² shown side by side. Apply the top entry to use it as the starting circuit.

Initial guesses

Filled in automatically from the spectrum. Override any value by typing into its field.

Frequency range

Typical: Full acquired range

Set in Data Processing. Points outside the range are left out of the fit.

Point selection

Exclude individual points (inductive high-frequency tails, a noisy low-frequency tail, outliers) by clicking or box-dragging them in the fit window, then fit again. The raw data is unchanged; the selection is not saved to the file.

Example

Fit a Randles + Warburg circuit to a coated-steel sample

Open the EIS spectrum in Studio and switch to the Nyquist view.
Click the Fit badge next to the EIS experiment in the right sidebar, then pick Randles + Warburg.
Accept the suggested initial guesses and press Fit.
The simulated impedance is drawn over the data.
Inspect the residual plot. A flat band around zero confirms the model fits.

Result. Rs, Rct, Cdl, and σ (Warburg coefficient) with standard errors and a χ² figure of merit.

In Sensitify Studio

Circuit fitting opens from the Fit badge next to any EIS experiment in Studio's right sidebar. Pick one of the 17 built-in models or compose your own, run the fit, and the simulated impedance is drawn over the data. The badge stays highlighted while a fit is applied.

Step-by-step instructions: Fit a circuit