Capacitive touch sensing system with interference rejection
View Patent ↗An interference tolerant capacitive touch sensor readout circuit having improved power and area efficiency is disclosed. Interference rejection for the capacitive touch sensing system is realized by transferring charge between a capacitive touch sensor and the readout circuit at frequencies outside bands where a level of interference is unacceptable. Improved power and area efficient come from the simplicity of the readout circuit which comprises a switched-capacitor integrator, a comparator, a digital accumulator and number of switches for driving a touch sensor and a capacitive feedback loop. The readout circuit is capable of interfacing with both self and mutual capacitance sensor to achieve compatibility with a larger collection of sensors and provides additional sensing and diagnostic functionalities.
1. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the capacitive touch sensor during a conversion cycle, the method comprising:
generating a driving signal, the driving signal driving the capacitive touch sensor during the conversion cycle;
alternately storing charge transferred from the capacitive touch sensor to a switched-capacitor integrator during the conversion cycle;
recording total charge accumulated in the switched-capacitor integrator during the conversion cycle; and
wherein the recording step further includes modifying the switched-capacitor integrator by a fixed amount of charge using a feedback loop after charge stored in at least one capacitor of the switched-capacitor integrator exceeds a threshold level.
2. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 1 , wherein the recording step further includes detecting an amount of charge stored in the at least one capacitor of the switched-capacitor integrator by comparing the switched-capacitor integrator output voltage to the threshold level.
3. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 1 , wherein the recording step further includes accumulating a number of times the charge is modified using the feedback loop.
4. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 1 , wherein the driving signal switches in one fixed or multiple time-multiplexed patterns for modulating current flow from the capacitive touch sensor to the switched-capacitor integrator.
5. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 4 , wherein the driving signal is tuned to avoid detected in-band interference by operating at one fixed or multiple time-multiplexed frequencies outside bands where the interference level is unacceptable.
6. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 5 , wherein interference signals located outside of the pass-band of the tuned driving signal are attenuated.
7. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 1 , wherein charge transferred from the capacitive touch sensor to the switched-capacitor integrator is stored in one of two capacitors of the switched-capacitor integrator depending on a positive/negative direction of current flow from the capacitive touch sensor to the switched-capacitor integrator.
8. The method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 1 , wherein the capacitive touch sensor is either a mutual capacitance sensor or a self-capacitance sensor.
9. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the capacitive touch sensor during a conversion cycle, the circuit comprising:
a driving signal generator,
a capacitive touch sensor, an output of the driving signal generator fed to an input of the capacitive touch sensor;
a switched-capacitor integrator connected to an output of the capacitive touch sensor;
a comparator connected to an output of the switched-capacitor integrator to detect charge exceeding a threshold level;
a feedback circuit operatively connected to an output of the comparator and an input of the switched-capacitor integrator, wherein, when the charge exceeds the threshold level, the feedback circuit includes modifying the switched-capacitor integrator by a fixed amount of charge using a feedback loop; and
an accumulator operatively connected to an output of the comparator to record total charge moved by the feedback circuit during the conversion cycle.
10. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 9 , wherein the driving signal generator comprises alternate phase operations of a reference voltage through a switch S 1 and ground through a switch S 2 .
11. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 9 , wherein the driving signal comprises a square wave.
12. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 9 , wherein the switched-capacitor integrator comprises a positive cycle branch having a first capacitor with two positive cycle switches (S 4 and S 5 ) set to close during a positive cycle of the driving signal to collect capacitive touch sensor charge in the first capacitor; a negative cycle branch having a second capacitor and two switches (S 6 and S 7 ) set to close during a negative cycle of the driving signal to collect capacitive touch sensor charge in the second capacitor, and an operational amplifier to provide gain.
13. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 9 , wherein the feedback loop includes a feedback digital to analog converter (DAC) comprising at least a third capacitor and a set of driving signal controlled switches (S 8 and S 9 ).
14. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 9 , wherein the capacitive touch sensor is either a mutual capacitive touch sensor or a self-capacitive touch sensor.
15. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 14 , wherein the circuit further comprises a switch (S 10 ) inserted between the capacitive touch sensor and the switched-capacitor integrator to enable use of either of the mutual capacitive touch sensor or the self-capacitive touch sensor.
16. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 9 , wherein the driving signal generator switches at one fixed or multiple time-multiplexed frequencies outside bands where a level of the interference is unacceptable.
17. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the capacitive touch sensor during a conversion cycle, the circuit comprising:
a driving signal generator comprising multiple modes;
a capacitive touch sensor, the capacitive touch sensor comprising either a mutual capacitive touch sensor or a self-capacitive touch sensor, an output of the driving signal generator operatively fed to an input of the capacitive touch sensor;
a switched-capacitor integrator connected to an output of the capacitive touch sensor;
a switch inserted between the capacitive touch sensor and the switched-capacitor integrator to sense either a mutual capacitive touch sensor in a first of the multiple modes or a self-capacitive touch sensor in a second of the multiple modes;
a comparator connected to an output of the switched-capacitor integrator to detect charge exceeding a threshold level;
a feedback circuit operatively connected to an output of the comparator and an input of the switched-capacitor integrator to modify charge from the switched-capacitor integrator when the threshold level is exceeded; and
an accumulator operatively connected to an output of the comparator to record total charge removed by the feedback circuit during the conversion cycle.
18. The circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle of claim 17 , wherein the feedback circuit comprises a feedback loop, the feedback loop including a feedback digital to analog converter (DAC) comprising at least a third capacitor and a set of driving signal controlled switches and wherein the feedback circuit includes modifying the charge by a fixed amount from the switched-capacitor integrator using the feedback loop the threshold level is exceeded.