Pressure sensing apparatus and method

Apparatus 22 and method for processing signals from a touch panel 10. The touch panel 10 includes a layer of piezoelectric material disposed between a plurality of sensing electrodes 14, 20 and at least one common electrode 15. The apparatus 22 includes a first circuit 23 for connection to the plurality of sensing electrodes 14, 20 configured to generate signals indicative of a pressure acting on the touch panel 10. A second circuit 24 for connected to the at least one common electrode is configured to generate a second pressure signal 30 indicative of a total pressure applied to the touch panel 10. A controller 25 is configured to determine an external interference signal 32 based on a weighted sum over the second pressure signal 30 and the plurality of first pressure signals 29. The controller 25 is also configured to compare the external interference signal 32 against a pre-calibrated threshold. If interference is above a threshold, the controller 25 outputs an interference flag indicating that the first and second pressure signals 29, 30 are influenced by coupling to one or more external electrical fields.