Stray field immunity

More about the effect of magnetic stray field and the differential sensing architecture.

The effect of magnetic stray fields

Magnetic stray fields may be generated by magnets, motors, transformers or any current-carrying conductors such as electric power lines. They are of growing concern in one sector of the automotive industry in particular: the number and the strength of the stray magnetic fields is greater in electric and hybrid-electric vehicles (EVs and HEVs) than in conventional cars powered by an internal combustion engine (ICE). Magnetic stray fields influence most position sensors, which are based on Hall effect technology.

ams OSRAM position sensors feature a unique patented technology which protects them against interference from magnetic stray fields.

Because the technology behind stray field immunity is integrated into the sensor IC, no external shielding needs to be mounted on the chip. This helps to lower the bill-of-materials and assembly cost of a position sensing system.

Differential sensing architecture

ams OSRAM position sensors offers system designers built-in immunity via differential sensing technology.

A position sensor such as the AS5147 has four integrated Hall elements in a north/east/south/west configuration. These elements are only sensitive to the Bz vector of the magnetic field and do not measure the Bx and By vectors. This means that they are inherently immune to stray magnetic field changes in the x and y dimensions.

The influence of an external field in the z-direction will be almost exactly the same on all four Hall elements; the sensor calculates the position based on the difference between the outputs of two pairs of Hall elements. Differential sensing can eliminate the influence of almost any external magnetic stray field.

Using a sensor based on the differential sensing principle offers various advantages:

  • No external components and no additional shielding needed
  • Higher system safety
  • Higher reliability
  • Cost effective sensor system
  • Small form factor

Stray field immunity

Stray Field Immunity is a unique patented technology by ams OSRAM, protecting sensors from interfering magnetic fields. Additional external shielding variants are not necessary because the SFI-technology is integrated into the IC, which decrease costs at the final application level.