Pseudo-differential phase measurement and quality factor compensation

A resistive-inductive-capacitive sensor is driven with first and second driving signals of different frequency f1, f2. A first value of a physical quantity e.g., amplitude of the sensor in response to the first driving signal, and a second value of the physical quantity in response to the second driving are measured 702, 704. Third and fourth values of the physical quantity are also measured at a later time 706, 708. A first difference between the third value and the first value, and a second difference between the fourth value and the second value, are determined. Based on the first and second differences, it is determined if a change in a resonant property of the sensor has occurred. The differences can be used to distinguish between electromagnetic interference 712, a change in quality factor of the sensor (720, figure 7B) or a human interaction with the sensor (718, figure 7B). The sensor may be in a user interface of a mobile device.