A computer based, four terminal impedance measuring system for low frequencies (designed for plasma membrane of living cells)

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Abstract
A four terminal high precision measuring system is described for the determination of capacitance, C, and conductance, G, at very low frequencies (0.1-1000 Hz). The method, which offers both speed (close to the theoretical upper limit) and accuracy (phase +or-0.01 degrees , impedance amplitude +or-0.1%) was specifically designed for the investigation of the dispersion of C and G in the plasma membrane of living cells. The system, with minor modifications should, however, also find application elsewhere. The system described was designed around a small computer which is used, via analogue-digital and digital-analogue converters and digital interfaces to both control and measure the frequency and amplitude of the AC signals and to measure the AC potential appearing across the impedance under test. After the data for a complete set of programmed frequencies has been collected the analysis of the data points for current and voltage, which may be accumulated in memory over a specified number of cycles, yields the relative phase and amplitude of the current and voltage and hence C and G.