optimized Impedance Measurement with AD5933

Summary: Some of the basic cases of analog-front-end electronic circuits for calibrated or direct impedance measurement with AD5933 presented in the literature are analyzed and compared. It was found that the separate measurement of the voltage drop on and the current flow through the sample avoids the calibration procedure, but does not essentially improve the measurement. The errors are mainly related to the usage of current-to-voltage convertor based on transimpedance amplifier. Its main disadvantages are the high Op-Amp input resistance and stray capacitance and the unbalanced input resistances. Significant reduction of the impedance magnitude/phase errors is achieved by direct measurement of the voltage and current with dual instrumentational amplifier. Such a circuit equalizes the time for the signal propagation in voltage measurement and current conversion modules. Moreover the high common mode rejection ratio and the very low input stray capacitance additionally improve the measurement. The impedance measurement with AD5933 based on the improved analog-front-end unit agreed well with results obtained by reference impedance analyzer.