Towards digital fundamental mode orthogonal fluxgate
Noise of orthogonal fluxgate in fundamental mode has been strongly reduced in the last years, making it a very competitive vectorial sensor of magnetic field at room temperature. The most important results have been achieved reducing the 1/f noise, which can reach about 1 pT/√Hz at 1 Hz. However, the noise floor is still an issue, which cannot be ignored anymore. The problem of the noise floor in fundamental mode orthogonal fluxgate became evident when sensors based on annealed wire-cores have been proposed. By annealing the wire with proper current flowing through it, the circumferential anisotropy is increased and therefore the 1/f noise reduced. However, the sensitivity is also reduced and that affects the noise floor, which is typically due to the white noise of analog demodulator. The challenge is to produce sensors with simultaneously low 1/f noise and low noise floor. For this reason we propose a transition to digital demodulation of the output voltage of the sensor. The voltage is digitized by high-speed high-resolution digitizer and the first harmonic extracted numerically. In this way we get rid of the noise of the analog demodulator. In this paper we prove that using this method we reduce the noise floor from 650 fT/√Hz to 400 fT/√Hz, a value which could never be achieved with analog demodulator, without increasing the 1/f noise. Moreover, we show how a digital demodulation allows us to efficiently compensate the noise in the output of the sensors due to excitation current. This result is achieved by simultaneous sampling of the excitation current and applying an algorithm based on its correlation to the output voltage of the sensor.