A device comprising: a processor configured to control the device; a touch panel comprising a layer of piezoelectric material disposed between a plurality of sensing electrodes (14, 20) and at least one common electrode; and an apparatus for processing signals from the touch panel, the apparatus comprising: a first circuit for connection to the plurality of sensing electrodes and configured to generate a plurality of first pressure signals, wherein each first pressure signal corresponds to one or more sensing electrodes and is indicative of a pressure acting on the touch panel proximate to the corresponding one or more sensing electrodes; a second circuit for connection to the at least one common electrode and configured to generate a second pressure signal indicative of a total pressure applied to the touch panel; a controller configured: to determine an external interference signal based on a weighted sum over the second pressure signal and the plurality of first pressure signals; and to determine a location at which pressure is applied to the touch panel; wherein the device is configured: to compare the external interference signal against a pre-calibrated threshold, and in response to the external interference signal being greater than or equal to the pre-calibrated threshold, to output an interference flag (Int flag) indicating that the first and second pressure signals are influenced by coupling to one or more external electrical fields; and in response to the interference flag being set, to process the first and/or second pressure signals using a signal post-processing method configured to reduce or remove the influence of coupling to one or more external electrical fields. a processor configured to control the device; a touch panel comprising a layer of piezoelectric material disposed between a plurality of sensing electrodes (14, 20) and at least one common electrode; and an apparatus for processing signals from the touch panel, the apparatus comprising: a first circuit for connection to the plurality of sensing electrodes and configured to generate a plurality of first pressure signals, wherein each first pressure signal corresponds to one or more sensing electrodes and is indicative of a pressure acting on the touch panel proximate to the corresponding one or more sensing electrodes; a second circuit for connection to the at least one common electrode and configured to generate a second pressure signal indicative of a total pressure applied to the touch panel; a controller configured: to determine an external interference signal based on a weighted sum over the second pressure signal and the plurality of first pressure signals; and to determine a location at which pressure is applied to the touch panel; a first circuit for connection to the plurality of sensing electrodes and configured to generate a plurality of first pressure signals, wherein each first pressure signal corresponds to one or more sensing electrodes and is indicative of a pressure acting on the touch panel proximate to the corresponding one or more sensing electrodes; a second circuit for connection to the at least one common electrode and configured to generate a second pressure signal indicative of a total pressure applied to the touch panel; a controller configured: to determine an external interference signal based on a weighted sum over the second pressure signal and the plurality of first pressure signals; and to determine a location at which pressure is applied to the touch panel; to determine an external interference signal based on a weighted sum over the second pressure signal and the plurality of first pressure signals; and to determine a location at which pressure is applied to the touch panel; wherein the device is configured: to compare the external interference signal against a pre-calibrated threshold, and in response to the external interference signal being greater than or equal to the pre-calibrated threshold, to output an interference flag (Int flag) indicating that the first and second pressure signals are influenced by coupling to one or more external electrical fields; and in response to the interference flag being set, to process the first and/or second pressure signals using a signal post-processing method configured to reduce or remove the influence of coupling to one or more external electrical fields. to compare the external interference signal against a pre-calibrated threshold, and in response to the external interference signal being greater than or equal to the pre-calibrated threshold, to output an interference flag (Int flag) indicating that the first and second pressure signals are influenced by coupling to one or more external electrical fields; and in response to the interference flag being set, to process the first and/or second pressure signals using a signal post-processing method configured to reduce or remove the influence of coupling to one or more external electrical fields.