A sensor device is provided. The sensor device includes a sensor pad for detecting an object in a detection area by measuring the impedance variation of the sensor pad, the sensor device further including a back focus plate disposed behind the sensor pad relative to the detection area and a first surrounding focus plate arranged around the sensor pad. The sensor device includes a signal generator that drives the sensor pad and the focus plates with a first set of signals having a predetermined constant phase, frequency and voltage amplitude. The sensor device further comprises at least a second surrounding focus plate arranged around the first surrounding focus plate, the second surrounding focus plate being driven with a second set of signals that are different from the first set of signals feeding the sensor pad.