A device includes a magnetic tunnel junction structure and a cap layer in contact with the MTJ structure. The power module assembly comprises a conductive base layer electrically coupled to the chassis, an insulating layer disposed on the conductive base layer, a first conductive node disposed on the insulating layer, a second conductive node disposed on the insulating layer, wherein the first and second conductive nodes are electrically isolated from each other. In order to be able to reliably detect even high-frequency deviations from the reference current, the measuring transformer includes, a reference setting unit for acting on the secondary winding with the reference current.