The primary-side voltage profile of an ignition voltage is converted into an analog current through the use of a first regulating circuit, whose output voltage is compared with a first comparison voltage in a first comparator. The output voltage is compared with a second comparison voltage in a second comparator. A diagnostic signal for the ignition switch can be tapped at the output of the first comparator, and a measurement signal for the burning duration can be tapped at the output of the second comparator.