Reduction of electromagnetic interference in a capacitive touchscreen system

A processor for a capacitive touchscreen system comprising a touchscreen comprising a first plurality of electrically conductive drive electrodes arranged in rows or columns, and a second plurality of electrically conductive, sense electrodes arranged in rows or columns arranged at an angle with respect to the rows or columns of the first plurality of electrodes, mutual capacitances existing between the first and second pluralities of electrodes at locations where the first and second pluralities of electrodes intersect, such mutual capacitances changing in the presence of one or more fingers or touch devices brought into proximity thereto, a system ground being associated with the capacitive touchscreen system, an electric field ground being associated with the one or more fingers or touch devices and the touchscreen, the processor comprising:

• at least one drive circuit operably connected to the first plurality of electrodes and configured to provide drive signals of high and low states thereto;
• a plurality of sense circuits, one each of the sense circuits being operably connected to corresponding ones of the second plurality of electrodes;
• wherein each of the sense circuits comprises a charge integrator circuit and a switched capacitance filtering circuit, the charge integrator circuit being configured to receive input signals provided by a corresponding one of the second plurality of electrodes and to provide integrated signals to the switched capacitance filtering circuit, the switched capacitance filtering circuit being configured to sample and store in first and second capacitors, respectively, first and second integrated signals delivered thereto by the charge integrator circuit where the first integrated signal corresponds to the high state drive signal, and the second integrated signal corresponds to the low state drive signal, the switched capacitance filtering circuit being configured to provide an output signal representative of the mutual capacitance of the one of the second plurality of electrodes corresponding to the sense circuit and further having an electromagnetic interference voltage between the system ground and the electric field ground substantially filtered therefrom by the twitched capacitance filtering circuit.

A method of reducing electromagnetic interference in a capacitive touchscreen system comprising a touchscreen comprising a first plurality of electrically conductive drive electrodes arranged in rows or columns, and a second plurality of electrically conductive sense electrodes arranged in rows or columns arranged at an angle with respect to the rows or columns of the first plurality of electrodes, mutual capacitances existing between the first and second pluralities of electrodes at locations where the first and second pluralities of electrodes intersect, such mutual capacitances changing in the presence of one or more fingers or touch devices brought into proximity thereto, a system ground being associated with the capacitive touchscreen system, an electric field ground being associated with the one or more fingers or touch devices and the touchscreen, the method comprising:

• sequentially driving the first plurality of electrodes with alternating high and low state drive signals;
• sensing the mutual capacitances associated with each of the second plurality of electrodes during the high and low state drive signals with a charge integrator circuit for each sense circuit;
• for each charge integrator circuit, providing first and second integrated signals corresponding to the high and low state drive signals, respectively, to a switched capacitance filtering circuit;
• sampling and storing in first and second capacitors, respectively, of the switched capacitance filtering circuit the first and second integrated signals;
• providing, with the switched capacitance filtering circuit, an output signal representative of the mutual capacitance of the one of the second plurality of electrodes corresponding to the sense circuit and further having an electromagnetic interference voltage between the system ground and the electric field ground substantially filtered therefrom by the switched capacitance filtering circuit.