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Scientific research on non-destructive testing is constantly looking for new solutions, about sensors and digital signal processing techniques, to improve the detection and characterization of defects in materials. The authors have engaged this particular line of research with particular reference to non-destructive testing applied to the conductive material through the use of eddy currents. After the realization of a novel instrument based on a Giant Magneto-Resistance probe, in this paper they propose the use of a suitable multi-frequency approach to improve reliability and accuracy in the defect estimation. Key features of the proposed instrument are the capability to detect, locate, and characterize thin defects as superficial and sub-superficial cracks. A number of tests carried out both in simulation and real environments show the goodness of the proposal.