Low-Frequency Magnetic Noise in Statically-Driven Solenoid for Biasing Magnetic Field Sensors

For the generation of static magnetic fields solenoids are frequently used for the purpose of research and development of magnetic field sensors. When such a sensor is to be analyzed with regard to its inherent noise the influence of other noise sources of the measurement system needs to be minimized. This article reports on the low-frequency magnetic noise within such a coil system. On the one hand, the impact of the intrinsic noise of the coil itself and on the other hand the impact of additional current noise of various commercially available current sources, which accordingly also leads to magnetic noise within the coil, are investigated. With low-frequency values in the range of a few tens of $\mathrm{fT}/\sqrt{\mathrm{Hz}}$, the coil's inherent noise is mostly neglectable. However, frequently utilized current sources for the generation of a static magnetic bias field lead to significant low-frequency magnetic flux noise typically in the $\mathrm{nT}/\sqrt{\mathrm{Hz}}$ regime. It is found that this noise cannot be decreased by increasing the coil's magnetic sensitivity, i.e. the magnetic flux density as a function of the static current. Instead, current sources with very high current-to-current-noise ratios are required.