A field device for determining and/or monitoring a physical or chemical, process variable in automation technology. The field device comprises: at least one microcontroller; a supply unit of limited capacity as primary voltage supply; and a detection circuit, which signals the microcontroller of a failure of the supply unit so early by means of a detection signal that sufficient time is available to take suitable safety measures, in order to avoid loss of data. The detection circuit is so embodied that a reference voltage required for detection of a voltage decline of the supply unit has an almost constant offset from the supply voltage of the supply unit and adjusts to the supply voltage, until the detection circuit, in the case of an abrupt subceeding of the reference voltage, or an abrupt voltage decline of the supply voltage, sends a detection signal to the microcontroller.