Reduction of noise in fundamental mode orthogonal fluxgates by optimization of excitation current

Reduction of noise is a major issue for orthogonal fluxgates, since the main drawback of this kind of sensor is higher noise than parallel fluxgates. Fundamental mode orthogonal fluxgates were successfully proposed to reduce the noise level, thanks to dc bias added to the excitation current. It has been shown that the main reason for noise suppression in fundamental mode orthogonal fluxgates is the reduction of Barkhausen noise obtained with a large dc bias, which makes the excitation current unipolar and avoids reversal of magnetization. Thus, one might believe that a low ac current and a large dc bias are always advantageous. In this paper we will show that this is not always true when this sensor is used in a magnetometer operated in feedback mode. For a given dc bias the 1/f noise will drop when we increase the ac current due to the increment of sensitivity inside the loop. The 1/f noise will reach a minimum level, and then if we further increase the ac current it will rise back up because the increment of Barkhausen noise becomes predominant over the increment of sensitivity. The minimum point is also verified by measurement of circumferential fluxcurrent loop. By properly choosing excitation current parameters, we achieved a 2.5 pT/√Hz noise floor and 7 pT/√Hz noise at 1 Hz. A general rule is finally proposed to easily minimize the noise.