DETERMINATION OF ELECTRODE IMPEDANCE WITH AN FFT SPECTRUM ANALYZER

The measurement of electrode impedances with an f. f. t. -spectrum analyzer is proved to be a fast and convenient method which could be automated by external control of the instrument (HP 3582A) via the IEEE-488 bus. Particular features are a large frequency span extending the analysis to frequencies below 1 Hz which are still in the range of physiological signals, the presentation of the results as a transfer function and validation of the results by means of the coherence function. In particular these features are most useful in sequential measurements of electrode impedance when studying the temporal variations of electrode characteristics. Furthermore, this technique enables the rapid detection and identification of sources of unwanted signals from a particular electrode configuration. This situation arises frequently as the driving signals in biomedical investigations must be kept fairly low due to nonlinear effects in the electrode/electrolyte interface and also becomes valuable when these instruments are used for the measurement of the low-frequency dispersion of impedance in an in-vitro or an in-vivo arrangement.