A magnetic field sensor (102; 502; 702; 900; 1202; 1302; 1402), comprising: a magnet (116; 506; 706; 1206; 1306; 1406) operable to generate a DC magnetic field; and first and second magnetic field sensing elements (114a,b; 602a,b; 802a,b; 902a,b; 1304a,b; 1404a,b) operable to sense first and second influenced magnetic fields, respectively, related to the DC magnetic field but influenced by at least one ferromagnetic surface (510a,b; 1210a,b; 1310a,b; 1410a,b) upon a ferromagnetic target object (104; 508; 708; 1208; 1308; 1408), wherein the first and second magnetic field sensing elements are disposed proximate to the at least one ferromagnetic surface, wherein the first and second magnetic field sensing elements are also operable to generate corresponding first and second electronic signals (1002, 1004) related to the first and second influenced magnetic fields, corresponding to a first and second ferromagnetic surface, respectively, wherein a difference of amplitudes of the first and second electronic signals is related to a relative location of the magnetic field sensor and the ferromagnetic target object along a movement axis, characterised in that a change of the relative location is indicative of a relative displacement of the magnetic field sensor and the ferromagnetic object, and wherein the difference of the amplitudes of the first and second electronic signals is not related to a rotation of the ferromagnetic target object about a rotation axis passing through the ferromagnetic target object, but is responsive only to a relative location of the ferromagnetic target object and the magnetic field sensor in a direction parallel to the rotation axis. a magnet (116; 506; 706; 1206; 1306; 1406) operable to generate a DC magnetic field; and first and second magnetic field sensing elements (114a,b; 602a,b; 802a,b; 902a,b; 1304a,b; 1404a,b) operable to sense first and second influenced magnetic fields, respectively, related to the DC magnetic field but influenced by at least one ferromagnetic surface (510a,b; 1210a,b; 1310a,b; 1410a,b) upon a ferromagnetic target object (104; 508; 708; 1208; 1308; 1408), wherein the first and second magnetic field sensing elements are disposed proximate to the at least one ferromagnetic surface, wherein the first and second magnetic field sensing elements are also operable to generate corresponding first and second electronic signals (1002, 1004) related to the first and second influenced magnetic fields, corresponding to a first and second ferromagnetic surface, respectively, wherein a difference of amplitudes of the first and second electronic signals is related to a relative location of the magnetic field sensor and the ferromagnetic target object along a movement axis, characterised in that a change of the relative location is indicative of a relative displacement of the magnetic field sensor and the ferromagnetic object, and wherein the difference of the amplitudes of the first and second electronic signals is not related to a rotation of the ferromagnetic target object about a rotation axis passing through the ferromagnetic target object, but is responsive only to a relative location of the ferromagnetic target object and the magnetic field sensor in a direction parallel to the rotation axis.