A device, in particular a pre-assembled transmission cable, is used to transmit differential data signals in a high-speed data connection. The device contains a circuit board with a conductor pair with two signal conductors for transmitting the differential data signal. An output line which is insulated from the conductor pair is paired with the conductor pair, the output line running parallel to the signal conductors in a non-interrupted manner and additionally being connected to a measuring conductor via at least one damping element. By virtue of the configuration, an undesired common-mode signal component is at least partly coupled into the output line, where the signal component is absorbed during operation.