A circuit is provided that comprises a transformer having a first coil, which is arranged on a substrate, a second coil, which is arranged above the first coil on the substrate, and a dielectric between the first coil and the second coil. The circuit furthermore comprises a resonant circuit, which is couplable to the first coil and/or the second coil to form a resonant loop, wherein a measure of a characteristic frequency of the resonant loop and/or a measure of a power consumption of the resonant loop is able to be tapped off at an output of the resonant circuit.
A corresponding method is also provided.