Angle sensing using differential magnetic measurement and a back bias magnet

A method of sensing an absolute relative rotation of a ferromagnetic target object and a magnetic field sensor about a rotation axis, the method comprising:

• generating a DC magnetic field by a back bias magnet;
• generating first and second electronic signals, respectively, by first and second magnetic field sensing elements disposed along a sensing element line and disposed proximate to at least one ferromagnetic surface of a ferromagnetic target object, the first and second electronic signals in response to first and second sensed magnetic fields corresponding to the DC magnetic field but influenced by the at least one ferromagnetic surface;
• generating, by the magnetic field sensor, a difference signal that is a difference of amplitudes of the first and second electronic signals, and the difference signal is indicative of a rotation measurement of an absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about a rotation axis, wherein the sensing element line is substantially parallel to the rotation axis, axis, wherein the at least one ferromagnetic surface of the ferromagnetic target object comprises a helical groove disposed on the ferromagnetic target object, the helical groove configured to provide the difference signal with respect to the absolute relative rotation.