Switchable attenuation of low magnetic fields for integrated vertical hall sensors using a ferromagnetic layer

The typical measurement range of integrated Hall sensors lies between approximately 10 mu T and 10 T. Above that range, nonlinear effects occur due to an increasing Hall angle. Below that range, sensor offsets and noise corrupt measurements. Because of its geometry, vertical Hall elements exhibit considerably less sensitivity making them more susceptible to offset and noise. We present a method of attenuating external fields by using a coating of ferromagnetic material for the purpose of offset calibration. If the coating is driven into saturation by an integrated excitation wire, the external field becomes "visible" for the sensor and can be measured offset-free. As the integrated excitation wire is able to saturate the coating very locally the unsaturated regions of it may act as a concentrator of the external field and hence increases the sensitivity. Different materials and geometries were modelled and simulated with the finite element method (FEM) and the approach was tested for feasibility. The results of the simulations were used to design an experimental integrated circuit (IC) and to choose a suitable material for the coating layer. With this IC we tested the new sensor and obtained first results which are presented in this paper.