Apparatus (1) for determining and/or monitoring at least one process variable, comprising: a primary side (I) and a secondary side (II), which are galvanically isolated from one another, wherein the secondary side (II) has a sensor element sensitive for the process variable (2) and an electronics unit (9) of the secondary side and provides a measurement signal representing the process variable, and wherein the primary side (I) has an electronics unit (8) of the primary side for evaluating the measurement signal and for producing an output signal. The apparatus (1) is distinguished by features including that the electronics unit (9) of the secondary side has a modulation unit (14), which produces a modulated measurement signal by at least at times modulating at least one other piece of information onto the measurement signal representing the process variable, and that the electronics unit (9) of the secondary side transmits the modulated measurement signal via a galvanically isolated interface to the primary side (I).