Offset-free bidirectional current source for impedance measurement

In order to minimize the influence of electrode polarization and noise in impedance measurements, especially in the low frequency region, current injection shows several advantages compared to voltage application. Although the existence of a great variety of current sources, they meet our requirements only partially. We developed a voltage controlled current source for active front ends for a wide range of impedances, from Ω up to TΩ. A broad bandwidth from dc to 100 MHz was ensured by minimizing the parasitic capacitances. The current source outputs a symmetric signal without dc-offset. This is accomplished by a differential driver where the non-inverting branch was used for creating a current conveyor while the inverting branch host the feedback for offset control. We focused especially on the stability of the current source for usage with small metal electrodes in aqueous solutions.