How can I tell that my EIS measurement is reliable using THD, NSD and NSR?
The three EIS quality indicators (EIS QI) in EC-Lab give linearity, stationarity and noise as a percentage at each frequency. The right excitation amplitude and the reliable frequency range are chosen with these indicators.
Key points
- THD (total harmonic distortion): the ratio of the harmonic amplitudes in the output signal to the amplitude at the fundamental frequency. In EC-Lab it is calculated for potential and current over 7 harmonics including the fundamental. Below 5% is generally considered linear.
- NSD (non-stationary distortion): the ratio of the components right next to the fundamental (f ± Δf), where Δf is the inverse of the total measurement time. It shows that the response has not reached steady state or that system parameters change with time.
- NSR (noise-to-signal ratio): the ratio of all energy outside the fundamental, the 7 harmonics and the adjacent frequencies; it reflects instrument accuracy and precision and external interference.
- Choosing the amplitude: large enough to minimise NSR, but small enough to keep THD below 5% at all frequencies.
- Choosing the frequency range: time variance usually appears below 1 Hz and NSD increases at low frequencies. If the time variance of the system cannot be reduced, data at frequencies where NSD exceeds 5% should be left out.
- 5% is an arbitrary threshold to be interpreted for the system and frequency. Detailed definitions and examples are in Bio-Logic AN64, AN65 and the 'Systems and EIS quality indicators' white paper.
Related application notes
- What do the EIS quality indicators THD, NSD and NSR tell us? Corrosion AN64
- Which parameters affect the THD value, and how does it compare with other linearity tests? Corrosion AN65
- White paper: Systems and EIS quality indicators WP EIS QI
- Why is EIS misinterpreted on time-varying systems? Corrosion AN55
Related curriculum module: Electrochemical Impedance Spectroscopy (EIS)
Source: Bio-Logic Learning Center, “How to make reliable Electrochemical Impedance Spectroscopy measurements with EIS QI™” · Source updated: November 18, 2024
Copyright of the images and the original text belongs to Bio-Logic SAS.
Copyright of the images and the original text belongs to Bio-Logic SAS.