A magnetic circuit assembly (12, 14) and proof mass assembly, the magnetic circuit assembly comprising: a magnet assembly defining a central axis, wherein the magnet assembly comprises: a pole piece; and a magnet underlying the pole piece, wherein the magnet comprises a relatively high coefficient of thermal expansion, CTE, material; and an excitation ring, comprising: a base, comprising: a platform layer underlying the magnet, wherein the platform layer comprises a relatively high CTE material that matches the CTE of the magnet; an upper base layer underlying the platform layer, wherein the upper base layer comprises a relatively low CTE material; and a lower base layer underlying the upper base layer, wherein the lower base layer comprises a relatively high CTE material; and an outer ring overlying the upper base layer and positioned around the magnet assembly, wherein the outer ring comprises a relatively low CTE material, wherein the outer ring is coupled to an outer radial portion of the proof mass assembly, and wherein each relatively high CTE material has a higher CTE than each relatively low CTE material, characterized in that a difference between a CTE of the proof mass assembly and at least one of a CTE of the relatively low CTE material of the outer ring or a CTE of the relatively low CTE material of the upper base layer is less than about 2 ppm/°C at room temperature. a magnet assembly defining a central axis, wherein the magnet assembly comprises: a pole piece; and a magnet underlying the pole piece, wherein the magnet comprises a relatively high coefficient of thermal expansion, CTE, material; and a pole piece; and a magnet underlying the pole piece, wherein the magnet comprises a relatively high coefficient of thermal expansion, CTE, material; and an excitation ring, comprising: a base, comprising: a platform layer underlying the magnet, wherein the platform layer comprises a relatively high CTE material that matches the CTE of the magnet; an upper base layer underlying the platform layer, wherein the upper base layer comprises a relatively low CTE material; and a lower base layer underlying the upper base layer, wherein the lower base layer comprises a relatively high CTE material; and an outer ring overlying the upper base layer and positioned around the magnet assembly, wherein the outer ring comprises a relatively low CTE material, wherein the outer ring is coupled to an outer radial portion of the proof mass assembly, and wherein each relatively high CTE material has a higher CTE than each relatively low CTE material, characterized in that a difference between a CTE of the proof mass assembly and at least one of a CTE of the relatively low CTE material of the outer ring or a CTE of the relatively low CTE material of the upper base layer is less than about 2 ppm/°C at room temperature. a base, comprising: a platform layer underlying the magnet, wherein the platform layer comprises a relatively high CTE material that matches the CTE of the magnet; an upper base layer underlying the platform layer, wherein the upper base layer comprises a relatively low CTE material; and a lower base layer underlying the upper base layer, a platform layer underlying the magnet, wherein the platform layer comprises a relatively high CTE material that matches the CTE of the magnet; an upper base layer underlying the platform layer, wherein the upper base layer comprises a relatively low CTE material; and a lower base layer underlying the upper base layer, wherein the lower base layer comprises a relatively high CTE material; and an outer ring overlying the upper base layer and positioned around the magnet assembly, wherein the outer ring comprises a relatively low CTE material, wherein the outer ring is coupled to an outer radial portion of the proof mass assembly, and wherein each relatively high CTE material has a higher CTE than each relatively low CTE material, characterized in that a difference between a CTE of the proof mass assembly and at least one of a CTE of the relatively low CTE material of the outer ring or a CTE of the relatively low CTE material of the upper base layer is less than about 2 ppm/°C at room temperature.