This paper deals with the characterization of fluxgate magnetometers that adopt a FeSiB microwire as magnetic core. The proposed device has a number of peculiarities that make this sensor very interesting. In fact this magnetometer affords the detection of weak magnetic targets with a very high spatial resolution and low power consumption. The readout strategy is based on the use of the “Residence Times Difference”; the DC magnetic field information is derived from the temporal positions of the output waveform spikes. In the paper an accurate device characterization is performed with particular regards to sensor resolution, sensitivity and power consumption; moreover, some comparisons between the FeSiB microwire fluxgate and a “ribbon” Magnetic Alloy FR4-fluxgate, previously developed by the authors, are presented. Finally exhaustive experimental characterization results demonstrate the possibility to use the microwire fluxgate as a low power system (with high spatial resolution) operating with a sinusoidal bias current of 5mA_pp.