Common mode noise suppression during hovering and proximity detection
In one embodiment, a touch sensor includes a touch screen having a plurality of electrodes and a controller. The controller is operable to detect that an object is in proximity to the touch screen by measuring capacitance values from a reference area of the touch screen, determining a reference capacitance value using the measured capacitance values from the reference area, measuring capacitance values from a main area of the touch screen, and suppressing noise from the measured capacitance values of the main area by subtracting the reference capacitance value from the measured capacitance values of the main area of the touch screen. The reference area includes two or more of the plurality of electrodes, and at least one of the electrodes of the reference area is electrically coupled to a voltage reference. The main area includes electrodes of the plurality of electrodes that are not in the reference area.
1. A device comprising:
a controller operable to detect that an object is in proximity to a touch screen comprising a plurality of electrodes, the controller configured to:
synchronously measure a plurality of capacitance values from a reference area of the touch screen and a plurality of capacitance values from a main area of the touch screen, wherein:
the reference area comprises a separate physical area on the touch screen from the main area;
the reference area comprises two or more of the plurality of electrodes, at least one of the electrodes of the reference area being electrically coupled to a voltage reference; and
the main area comprises at least two or more electrodes of the plurality of electrodes that are not in the reference area;
determine a reference capacitance value using the plurality of measured capacitance values from the reference area; and
suppress common mode noise from the plurality of measured capacitance values of the main area of the touch screen by subtracting a capacitance value from each of the plurality of measured capacitance values of the main area of the touch screen, the capacitance value being based on the reference capacitance value.
2. The device of claim 1 , wherein electrodes of the reference area that are electrically coupled to the voltage reference are interleaved with electrodes of the reference area that are not coupled to the voltage reference.
3. The device of claim 1 , wherein the plurality of electrodes of the touch screen comprises a plurality of x-electrodes and a plurality of y-electrodes.
4. The device of claim 1 , wherein determining the reference capacitance value using the plurality of measured capacitance values from the reference area comprises calculating a mean capacitance value of the plurality of measured capacitance values from the reference area.
5. The device of claim 1 , wherein determining the reference capacitance value using the plurality of measured capacitance values from the reference area comprises calculating a minimum or a maximum capacitance value of the plurality of measured capacitance values from the reference area.
6. The device of claim 1 , wherein detecting that the object is in proximity to the touch screen further comprises multiplying the determined reference capacitance value by a scaling coefficient.
7. The device of claim 1 , wherein the controller is further operable to determine, based on the noise-suppressed capacitance values of the main area of the touch screen, a distance between the object and the touch screen.
8. A method comprising:
by a controller of a touch screen, synchronously measuring a plurality of capacitance values from a reference area of the touch screen and a plurality of capacitance values from a main area of the touch screen, wherein:
the reference area comprises a separate physical area on the touch screen from the main area;
the reference area comprises two or more of a plurality of electrodes of the touch screen, at least one of the electrodes of the reference area being electrically coupled to a voltage reference; and
the main area comprises at least two or more electrodes of the plurality of electrodes that are not in the reference area;
by the controller of the touch screen, determining a reference capacitance value using the plurality of measured capacitance values from the reference area; and
by the controller of the touch screen, suppressing common mode noise from the plurality of measured capacitance values of the main area of the touch screen by subtracting a capacitance value from each of the plurality of measured capacitance values of the main area of the touch screen, the capacitive value being based on the reference capacitive value.
9. The method of claim 8 , wherein electrodes of the reference area that are electrically coupled to the voltage reference are interleaved with electrodes of the reference area that are not coupled to the voltage reference.
10. The method of claim 8 , wherein the voltage reference comprises ground.
11. The method of claim 8 , wherein determining the reference capacitance value using the plurality of measured capacitance values from the reference area comprises calculating a mean capacitance value of the plurality of measured capacitance values from the reference area.
12. The method of claim 8 , wherein determining the reference capacitance value using the plurality of measured capacitance values from the reference area comprises calculating a minimum or a maximum capacitance value of the plurality of measured capacitance values from the reference area.
13. The method of claim 8 , further comprising multiplying the determined reference capacitance value by a scaling coefficient.
14. The method of claim 8 , further comprising determining, based on the noise-suppressed capacitance values of the main area of the touch screen, a distance between the object and the touch screen.
15. A touch-sensitive device comprising a controller, the controller operable to:
measure a plurality of capacitance values from each of a first, second, third, and fourth reference area of a touch screen, wherein:
at least one electrode of each reference area is electrically coupled to a voltage reference;
the first and second reference areas each comprise two or more of a plurality of x-electrodes of the touch screen, the first reference area being on an opposite side of the touch screen from the second reference area; and
the third and fourth reference areas each comprise two or more of a plurality of y-electrodes of the touch screen, the third reference area being on an opposite side of the touch screen from the fourth reference area;
measure a plurality of capacitance values from each of a first, second, third, and fourth main area of the touch screen, wherein:
the first, second, third, and fourth reference areas each comprise a separate physical area on the touch screen, respectively, from the first, second, third, and fourth main areas;
capacitance values from the first main area are measured synchronously with the capacitance measurements of first reference area;
capacitance values from the second main area are measured synchronously with the capacitance measurements of second reference area;
capacitance values from the third main area are measured synchronously with the capacitance measurements of third reference area;
capacitance values from the fourth main area are measured synchronously with the capacitance measurements of fourth reference area;
determine a first, second, third, and fourth reference capacitance value using the plurality of measured capacitance values from the first, second, third, and fourth reference areas, respectively; and
suppress common mode noise from the plurality of measured capacitance values of the first, second, third, and fourth main areas of the touch screen by subtracting, respectively, first, second, third, and fourth capacitance values from each of the plurality of measured capacitance values of the first, second, third, and fourth main areas of the touch screen, the first, second, third, and fourth capacitance value being based on the first, second, third, and fourth reference capacitance values, respectively.
16. The touch-sensitive device of claim 15 , wherein electrodes of the first, second, third, and fourth reference areas that are electrically coupled to the voltage reference are interleaved, respectively, with electrodes of the first, second, third, and fourth reference areas that are not electrically coupled to the voltage reference.
17. The touch-sensitive device of claim 15 , wherein determining the first, second, third, and fourth reference capacitance values comprises calculating a mean capacitance value of the first, second, third, and fourth measured capacitance values from the first, second, third, and fourth reference areas, respectively.
18. The touch-sensitive device of claim 15 , wherein determining the first, second, third, and fourth reference capacitance values comprises calculating a maximum or minimum capacitance value of the first, second, third, and fourth measured capacitance values from the first, second, third, and fourth reference areas, respectively.
19. The touch-sensitive device of claim 15 , the controller further operable to multiply the determined reference capacitance values by a scaling coefficient.
20. The touch-sensitive device of claim 15 , the controller further operable to determine, based on the noise-suppressed capacitance values of the main area of the touch screen, a distance between the object and the touch screen.
21. The device of claim 6 , wherein the scaling coefficient has a value less than one.
22. The device of claim 6 , wherein the scaling coefficient has a value greater than one.
23. The device of claim 6 , wherein a value of the scaling coefficient is a function of a signal amplitude of one or more measurement signals.