A multi-period interface system for impedance measurements

A novel multi-period interface system for impedance measurement, based on a first-order oscillator, is presented. The system is designed for sterility testing of aseptically packed food products by noninvasive measurement of the conductivity of the packaged food. In this application the measured impedance can be modeled as a resistor, representing the conductivity of the food in series with a capacitor, representing the electrical behavior of the walls of the food container. The measurement range is: 50 pF to 220 pF for the capacitive component and 10 Ω to 150 Ω for the resistive component. By applying auto-calibration, a relative error of 0.3% for the resistive component R _x and 0.1% error for the capacitive component C_x is achieved. The output signal is a period-modulated square wave signal, which can be processed directly by a microcontroller, without any additional interface circuit. To calculate the values for R_x and C_x, a data processing algorithm has been developed. The details of a prototype design have been discussed together with the experimental results