Described is monitoring of an analog-digital conversion of a measured value of at least one of a pressure gauge, a level gauge and a flowmeter. Different measured values can be provided for the analog-digital converter by means of a first microcontroller, wherein these measured values are subsequently transmitted from this first area into a second area by means of a unidirectional coupler. These values are acquired by a second microcontroller in the second area and compared with reference values that are stored in the second area or at another location. This diagnostic circuit may make it possible to detect drifts of resistances and of reference voltages, as well as a faulty analog-digital converter or a program execution error of the first microcontroller.