An optical converter is to be simply and reliably monitored. Thereto, a circuit device including a measurement bridge with a measurement branch, a first comparison branch and a second comparison branch is provided. A conductor path loop of the converter can be integrated in the measurement branch. The first comparison branch has a first series resistance variable via a first active element. Similarly, the second comparison branch has a second series resistance variable via a second active element. The first and the second active elements are respectively controllable by the same control signal. The circuit device further has an evaluation circuit with a first comparator and a second comparator, the outputs of which form the control signal and at the same time an output signal of the circuit device in AND-gated manner. Each comparator compares a tap of its respective comparison branch to a tap of the measurement branch.