Interface circuit for impedance measurement to test sterility of food products

A simple interface circuit for impedance measurement based on a second-order harmonic oscillator is presented It is intended for sterility testing of aseptically packed food products by measuring the conductivity changes of the packaged food in non-destructive way. The measured impedance is modeled as a resistor (representing the conductivity of the food) in series with a capacitor (due to the walls of the food container). For building the oscillator fast current-feedback opamp´s are used. To measure the resistive component of the impedance an AC-to-DC converter is realized with a wide frequency range analogue multiplier. The presented experimental results prove that with this interface circuit an accurate measurement of the impedance components can be achieved in a frequency range up to 10 MHz