An apparatus is described, including: a signal processing circuit adapted to process an input signal to obtain an output signal; a sensor element for sensing a predetermined physical quantity, wherein the sensor element is adapted to generate a sensor signal in response to the predetermined physical quantity; wherein the signal processing unit is adapted to process the input signal to obtain the output signal depending on the sensor signal; and wherein the apparatus further comprises an evaluation circuit adapted to evaluate the sensor signal and to generate an indication signal indicating an abnormal operating condition in case the sensor signal does not fulfill a predetermined normal operation criterion.