Methods and circuits for measuring mutual and self capacitance
View Patent ↗Capacitance measurement circuits for measuring self and mutual capacitances are described. In one embodiment the capacitance measurement circuit includes: a first electrode capacitively coupled with a second electrode; a first plurality of switches coupled with the first electrode; and a second plurality of switches coupled with the second electrode, wherein, during a first operation stage, the first plurality of switches is configured to apply a first initial voltage to the first electrode and the second plurality of switches is configured to apply a second initial voltage to the second electrode, and wherein, during a second operation stage, the first plurality of switches is configured to connect the first electrode with a measurement circuit, and the second plurality of switches is configured to connect the second electrode with a constant voltage.
1. A capacitive measurement circuit, comprising:
a first electrode capacitively coupled with a second electrode;
a first plurality of switches coupled with the first electrode; and
a second plurality of switches coupled with the second electrode, wherein, during a first operation stage, the first plurality of switches is configured to apply a first initial voltage to the first electrode and the second plurality of switches is configured to apply a second initial voltage to the second electrode, and wherein, during a second operation stage, the first plurality of switches is configured to connect the first electrode with a measurement circuit, and the second plurality of switches is configured to connect the second electrode with a constant voltage,
wherein the measurement circuit is configured to generate an output signal based on a current generated by operation of the first plurality of switches, the circuit generating the output signal including a current mirror with a first end and a second end, the first end coupled to the first plurality of switches.
2. The measurement circuit of claim 1 , wherein the circuit generating the output signal based on the current generated by the operation of the first plurality of switches further comprises:
a charge integration circuit coupled to the second end of the current mirror;
a comparator coupled to the charge integration circuit at a first comparator input and a reference voltage on a second comparator input; and
a timer coupled to an output of the comparator.
3. The measurement circuit of claim 2 , wherein an output of the timer is equivalent to the current generated by the operation of the first plurality of switches.
4. The capacitance measurement circuit of claim 1 , wherein the second initial voltage is approximately equal to the first initial voltage.
5. The capacitance measurement circuit of claim 1 , wherein the first initial voltage is approximately equal to a voltage supplied by the measurement circuit to the first electrode during the second operation stage.
6. The capacitance measurement circuit of claim 5 , further comprising a buffer configured to generate the first initial voltage based on the voltage supplied by the measurement circuit.
7. A capacitance measurement method, comprising:
applying a first initial voltage to a first electrode during a first operation stage, wherein the first electrode is capacitively coupled with a second electrode;
applying a second initial voltage to the second electrode during the first operation stage;
connecting the first electrode with a measurement circuit during a second operation stage;
connecting the second electrode to a constant voltage during the second operation stage; and
generating an output signal based on a net current detected by the measurement circuit during the first operation stage and the second operation stage, wherein the output signal is generated by a current mirror coupled to the current detected by the measurement circuit and coupled to a charge integration circuit coupled to a comparator and a timer.
8. The capacitance measurement method of claim 7 , wherein the second initial voltage is approximately equal to the first initial voltage.
9. The capacitance measurement method of claim 7 , wherein the first initial voltage is approximately equal to a voltage supplied by the measurement circuit to the first electrode during the second operation stage.
10. The capacitance measurement method of claim 9 , further comprising buffering the voltage supplied by the measurement circuit to generate the first initial voltage.
11. A capacitance measurement circuit, comprising:
a first electrode capacitively coupled with a second electrode;
a first plurality of switches coupled with the first electrode; and
a second plurality of switches coupled with the second electrode, wherein the first plurality of switches is configured to connect the first electrode with a measurement circuit during a first operation phase and to connect the first electrode to a constant voltage during a second operation phase, and wherein the second plurality of switches is configured to minimize a voltage difference between the second electrode and the first electrode during the first operation phase and during the second operation phase.
12. The capacitance measurement circuit of claim 11 , wherein the measurement circuit is configured to generate an output signal based on a current generated by operation of the first plurality of switches.
13. The measurement circuit of claim 12 , wherein the output signal based on the current generated by the operation of the first plurality of switches is generated by a circuit comprising:
a current mirror with a first end and a second end, the first end coupled to the first plurality of switches;
a charge integration circuit coupled to the second end of the current mirror;
a comparator coupled to the charge integration circuit at a first comparator input and a reference voltage on a second comparator input; and
a timer coupled to an output of the comparator.
14. The measurement circuit of claim 13 , wherein an output of the timer is equivalent to the current generated by the operation of the first plurality of switches.
15. The capacitance measurement circuit of claim 11 , wherein the second plurality of switches is further configured to:
during the first operation phase, connect the second electrode to a voltage approximately equal to a voltage supplied by the measurement circuit to the first electrode; and
during the second operation phase, connect the second electrode to a voltage approximately equal to the constant voltage.
16. The capacitance measurement circuit of claim 15 , further comprising a buffer configured to generate the voltage approximately equal to the voltage supplied by the measurement circuit to the first electrode.
17. The capacitance measurement circuit of claim 11 , wherein the constant voltage is a ground voltage.
18. A capacitance measurement method, comprising:
connecting a first electrode to a measurement circuit during a first operation phase, wherein the first electrode is capacitively coupled to a second electrode;
connecting the first electrode to a constant voltage during a second operation phase; and
minimizing a voltage difference between the second electrode and the first electrode during the first operation phase and the second operation phase.
19. The capacitance measurement method of claim 18 , further comprising generating an output signal based on a net current detected by the measurement circuit during the first operation phase and the second operation phase.
20. The capacitance measurement method of claim 19 , wherein the output signal is generated by a current mirror coupled to the current detected by the measurement circuit and coupled to a charge integration circuit coupled to a comparator and a timer.
21. The capacitance measurement method of claim 18 , wherein minimizing the voltage different comprises:
during the first operation phase, connecting the second electrode to a voltage approximately equal to a voltage supplied by the measurement circuit to the first electrode; and
during the second operation phase, connecting the second electrode to a voltage approximately equal to the constant voltage.
22. The capacitance measurement method of claim 21 , further comprising using a buffer connected to the measurement circuit to generate the voltage approximately equal to the voltage supplied by the measurement circuit to the first electrode.
23. The capacitance measurement method of claim 18 , wherein the constant voltage is a ground voltage.