A capacitive sensor device and method is configured to respond a stimulus provided in a sensing region with an output signal. A signal generator is configured to apply a carrier signal to the capacitive sensor device. The carrier signal is switched between a plurality of phases at a switching rate, where the switching rate is less than a demodulation filter bandwidth. The result of the carrier phase shifting is that effects of interference in the output signal are frequency shifted away from the effects of user applied stimulus. An interference detection filter is configured to filter from the sensor outputs at least one effect produced by the stimulus. An interference measuring device is configured to determine a level of interference in the at least one interference output. Thus, the system can detect interference in the output of the capacitive sensor device.