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The Giant Magneto-Inductive (GMI) effect in amorphous magnetic and conducting wires is analyzed using the concepts and words of electronics engineering to show the way high-sensitivity GMI magnetometers may be designed. Starting from a simple modeling of the magneto-impedance, direct and field-locked loop magnetometers are discussed, together with the implementation of a low-frequency noise-reduction technique that makes good use of a basic modulating method. It allows the removal of a part of the 1/f noise in the magnetometer. The unmodulated magnetometer characteristics are the following: bandwidth higher than 100 kHz and white noise level lower than 7 pT/√Hz above 40 KHz. Similarly, the main auxiliary modulated magnetometer characteristics are as follows: bandwidth of 4.8 kHz and white noise level lower than 60 pT/√Hz above 3 Hz. The slew rate in both cases is limited by the electronics to 5 mT/s. The dynamic of these magnetometers is about ±25 μT, which corresponds to more than 120 dB in 1 Hz bandwidth above 1 Hz.