According to the invention, the calibration measuring cycle is divided into several, particularly a plurality of partial cycles, with which one or more of the calibration measurements are associated. The drive circuit also has a first transistor with either a drain or a source terminal connected to the first voltage supply, and a second transistor with either a drain or source terminal connected to the second voltage supply. A metering device is provided between the delivery pump and the exhaust tube, which metering device supplies reducing agent, which is delivered continuously by the delivery pump, in an intermittent fashion into the exhaust strand tube.