A magnetic field sensor system, comprising: a ferromagnetic target object; a magnetic field sensor, whereby the magnetic field sensor comprises: a back bias magnet configured to generate a DC magnetic field; and first and second magnetic field sensing elements (902a, 902b); wherein the first and second magnetic field sensing elements (902a, 902b) are disposed along a sensing element line and configured to generate first and second electronic signals, respectively, in response to first and second sensed magnetic fields corresponding to the DC magnetic field but influenced by at least one ferromagnetic surface of the ferromagnetic target object, disposed proximate to the first and second magnetic field sensing elements; wherein the magnetic field sensor is operable to generate 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, when the at least one ferromagnetic surface of the ferromagnetic target object comprises a first surface having a first radius and a second surface having a second different radius, the first and second surfaces disposed on the ferromagnetic target object, the second surface displaced from the first surface in a direction along the rotation axis wherein the first radius and the second radius have respective decreasing spirals; a first distance between the first surface and the first magnetic field sensing element decreases while a second distance between the second surface and the second magnetic field sensing element increases in response to changes of the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor; and the first radius and the second radius are selected to provide the difference signal that is substantially linear with respect to the rotation of the ferromagnetic target object about the rotation axis. a ferromagnetic target object; a magnetic field sensor, whereby the magnetic field sensor comprises: a back bias magnet configured to generate a DC magnetic field; and first and second magnetic field sensing elements (902a, 902b); a back bias magnet configured to generate a DC magnetic field; and first and second magnetic field sensing elements (902a, 902b); wherein the first and second magnetic field sensing elements (902a, 902b) are disposed along a sensing element line and configured to generate first and second electronic signals, respectively, in response to first and second sensed magnetic fields corresponding to the DC magnetic field but influenced by at least one ferromagnetic surface of the ferromagnetic target object, disposed proximate to the first and second magnetic field sensing elements; wherein the magnetic field sensor is operable to generate 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, when the at least one ferromagnetic surface of the ferromagnetic target object comprises a first surface having a first radius and a second surface having a second different radius, the first and second surfaces disposed on the ferromagnetic target object, the second surface displaced from the first surface in a direction along the rotation axis wherein the first radius and the second radius have respective decreasing spirals; a first distance between the first surface and the first magnetic field sensing element decreases while a second distance between the second surface and the second magnetic field sensing element increases in response to changes of the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor; and the first radius and the second radius are selected to provide the difference signal that is substantially linear with respect to the rotation of the ferromagnetic target object about the rotation axis.