The invention relates to a device, in particular a pre-assembled transmission cable, which is used to transmit differential data signals (SD) in a high-speed data connection. The device comprises a circuit board (8) with a conductor pair with two signal conductors (14) for transmitting the differential data signal (SD). An output line (24) which is insulated from the conductor pair is paired with the conductor pair, said output line running parallel to the signal conductors (14) in a non-interrupted manner and additionally being connected to a measuring conductor (26) via at least one damping element (36). By virtue of said design, an undesired common-mode signal component is at least partly coupled into the output line (24), where the signal component is absorbed during operation.