A scanning unit is for scanning a measuring standard having a coded track formed by a graduated scale, such as an incremental track, and a reference mark system, including a detector system for scanning the coded track and an additional detector system for scanning the reference mark system. The detector system, when scanning the associated coded track or reference mark system, receives scanning signals on a signal-sensitive surface, and the additional detector system includes at least two sensors, of which only one is used for scanning the reference mark system during operation of the scanning unit. Each of the two sensors is connected to one of the two inputs of a differential amplifier, and the sensor not used for scanning is deactivated in that its signal-sensitive surface is covered.