Trackpad sensitivity compensation
A device compensating for a loss in sensitivity of a sensor, such as a trackpad. The device may be standalone, integrated with the trackpad itself, or within the associated integrated circuit designed to measure relative electrode capacitances. The embodiment generally employs a current source to negatively compensate for the current available to charge at least one capacitor (or other storage device) in the trackpad.
1. A sensor circuit, comprising:
at least a first electrode;
a storage device operatively connected to the first electrode;
a source operative to charge at least the first electrode; and
a current source operatively connected to the storage device and sinking a portion of a current while a remainder of the current is charging the storage device; wherein
the storage device is charged from the first electrode.
2. The sensor circuit of claim 1 , wherein the storage device is a capacitor.
3. The sensor circuit of claim 1 , further comprising:
a surface overlaying the at least one electrode; wherein
the surface is operative to capacitively couple a finger touching the surface to the first electrode.
4. The sensor circuit of claim 3 , wherein the tracking sensor operates in at least one dimension.
5. The sensor circuit of claim 1 , wherein the sensor circuit is a tracking sensor.
6. The sensor circuit of claim 1 , wherein the sensor is a touch button.
7. The sensor circuit of claim 6 , wherein the tracking sensor operates in two dimensions.
8. The sensor circuit of claim 1 , wherein the current source comprises a switched capacitor.
9. The sensor circuit of claim 1 , wherein the current source comprises a switched resistor.
10. The sensor circuit of claim 1 , wherein the current source comprises a current follower.
11. The sensor circuit of claim 1 , wherein the sensor circuit comprises an active current sink.
12. The sensor circuit of claim 1 , further comprising a comparator operatively connected to the storage device.
13. The sensor circuit of claim 12 , further comprising a switch operatively connected to the storage device.
14. The sensor circuit of claim 13 , wherein:
the comparator is operative to close the switch when the storage device reaches a preset voltage; and
the switch is operative to drain a voltage from the storage device when the switch closes.
15. The sensor circuit of claim 14 , further comprising:
a first switch connected between the first electrode and the voltage source; and
a second switch connected between the first electrode and second electrode; wherein
the opening and closing of the first and second switches is operative to charge the first and second electrodes at a given frequency.
16. A method for determining a presence of a finger, comprising:
charging a storage capacitor for a first time while a first voltage is negatively compensated by a current sink by sinking a portion of a current while a remainder of the current charges the storage capacitor;
discharging the storage capacitor;
charging the storage capacitor for a second time;
comparing the first time to the second time; and
in the event the second time is less than the first time, concluding a finger is present.
17. The method of claim 16 , wherein the step of charging a capacitor a first time comprises:
charging the storage capacitor from a current generated by a sense capacitor at a given frequency.
18. The method of claim 17 , wherein the step of charging the storage capacitor a second time comprises:
charging the storage capacitor from a current generated by a sense capacitor at a given frequency; and
negatively compensating the current before the current charges the storage capacitor.
19. The method of claim 18 , further comprising charging the sense capacitor at a given frequency from a voltage source.
20. The method of claim 19 , wherein the application of the negative compensation to the current increases a sensitivity of the sense capacitor to the presence of a finger.
21. The method of claim 20 , further comprising:
determining whether a voltage of the storage capacitor exceeds a threshold voltage;
in the event the voltage of the storage capacitor exceeds the threshold voltage, closing a switch; and
in response to closing the switch, resetting the voltage of the storage capacitor.
22. The method of claim 21 , wherein:
the operation of negatively compensating the current comprises:
connecting the current source and the storage capacitor to a common node; and
draining at least a portion of the current from the common node such that the drained portion of the current cannot charge the storage capacitor.
23. The method of claim 16 , wherein the current is negatively compensated by applying a current source to the current.