Low Offset Drift–Low-Noise Orthogonal Fluxgate With Synchronized Polarity Flipping

In this paper, we present an orthogonal fluxgate in fundamental mode with extremely stable offset and low noise. The typical solution to stabilizing the offset in this type of sensor is to flip the dc bias of its excitation current and subtract the output obtained with opposite polarities. In this case, on the contrary, we flip both the ac and dc components of the excitation current and sum the output voltages. This is performed using fast solid-state switches with low on-resistance. In this way, we achieved a large suppression of the dependence of the offset on temperature, obtaining an average offset drift of 0.012 nT/K, making this sensor on the top class for offset stability. We digitally sampled and numerically demodulated the output voltage of the sensor to extract the first harmonic. The value of the first harmonic shows noise right after switching of the current. Therefore, we numerically erased those few noisy values of the first harmonic before integrating it. In this way, we managed to limit the noise growth given by switching of the current. Finally, we strongly improved the offset stability with minor increment of noise.