Time and Frequency Domains and Transforms

Signal-processing theory refers to data domains. The same data can be represented in different domains. In EIS, we use two of these domains, the time domain and the frequency domain.

In the time domain, signals are represented as signal amplitude versus time.  
In the frequency domain, signals are represented as signal amplitude versus frequency.  

Use the Fourier transform and inverse Fourier transform to switch between the domains. The common term, FFT, refers to a fast, computerized implementation of the Fourier transform. Detailed discussion of these transforms is beyond the scope of this help file. Several of the references given at the end of this section contain more information on the Fourier transform and its use in EIS.

In modern EIS systems, lower-frequency data are usually measured in the time domain. The controlling computer applies a digital approximation to a sine wave to the cell by means of a digital-to-analog converter. The current response is measured using an analog-to-digital computer. An FFT converts the current signal into the frequency domain.