Apparatus (22) for processing signals from a touch panel (10) is described. The touch panel (10) includes a layer of piezoelectric material (16) 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). The first circuit (23) is configured to generate a plurality of first pressure signals (29). Each first pressure signal (29) corresponds to one or more sensing electrodes (14, 20) and is indicative of a pressure acting on the touch panel (10) proximate to the corresponding one or more sensing electrodes (14, 20). The apparatus (22) also includes a second circuit (24) for connection to the at least one common electrode (15). The second circuit (24) is configured to generate a second pressure signal (30) indicative of a total pressure applied to the touch panel (10). The apparatus (22) also includes a controller (25) 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 (Vthresh). The controller is also configured, in response to the external interference signal (32) being greater than or equal to the pre-calibrated threshold (Vthresh), to output an interference flag (Intflag) indicating that the first and second pressure signals (29, 30) are influenced by coupling to one or more external electrical fields.