A control device for controlling a supply voltage which can be switched between a constant line current operation and a constant line voltage operation, wherein a line current, which consists of a direct current for supplying the terminal and of an alternating current for transmitting information, flows via a subscriber line of a terminal, comprising a sensing circuit for sensing an analogue input voltage dependent on the line current, a subtractor which, in the constant line current operation, subtracts an analogue feedback voltage dependent on an adjustable nominal direct-current value from the sensed input voltage for generating an analogue difference voltage, an analogue/digital converter for converting the generated analogue difference voltage into a digital difference voltage value, a digital low-pass filter for filtering a sequence of control error values out of the generated sequence of difference voltage values, a controller for generating a control value in dependence on the filtered-out sequence of control error values, and comprising a first digital/analogue converter which, in the constant line current operation, converts the control value in each case generated by the controller into the analogue direct voltage for supplying the terminal.