METHODS AND SYSTEMS FOR SIGMA DELTA CAPACITANCE MEASURING USING SHARED COMPONENT

• US 20080061800A1
• Filed: 10/26/2007
• Published: 03/13/2008
• Est. Priority Date: 06/03/2005
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: Reynolds, Joseph, Hargreaves, Kirk
• Granted Patent: US 7,777,501 B2
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960725 Charge-transfer

Abstract

Methods, systems and devices are described for detecting a measurable capacitance using sigma-delta charge transfer techniques that can be implemented with many standard microcontrollers, and can share components to reduce device complexity and improve performance. In the various implementations of this embodiment, the passive network used to accumulate charge can be shared between multiple measurable capacitances. A switch or IO controlling the charge sharing and/or charge changing can also be shared Likewise, in various implementations a voltage conditioning circuit configured to provide a variable reference voltage can be shared between multiple measurable capacitances. Finally, in various implementations a guarding electrode configured to guard the measurable capacitances can be shared between multiple measurable capacitances. In each of these cases, sharing components can reduce device complexity and improve performance.

Claims

1. A system for measuring each of a plurality of measurable capacitances, the system comprising:

at least one passive network coupled to the plurality of measurable capacitances, each of the at least one passive network comprising at least one integrating capacitance configured to accumulate charge received from the plurality of measurable capacitances;

at least one charge changing circuit coupled to the at least one passive network; and

a controller, the controller including a plurality of digital outputs, each of the plurality of digital outputs coupled to a measurable capacitance in the plurality of measurable capacitances, the controller configured to measure each of the plurality of measurable capacitances by, for each selected measurable capacitance in the plurality of measurable capacitances, repeatedly applying a voltage to the selected measurable capacitance using a digital output in the plurality of digital outputs, repeatedly sharing charge between the selected measurable capacitance and the at least one passive network to accumulate charge on the at least one integrating capacitance, and repeatedly changing a charge on the at least one integrating capacitance by a quantized amount of charge using the at least one charge changing circuit in response to the charge on the at least one passive network being past a threshold of a quantizer, the quantized amount of charge determined at least in part as a function of an output of the quantizer.

2. The system of claim 1 wherein the controller is further configured to measure at least two of the plurality of measurable capacitances using a same integrating capacitance of the at least one passive network.

3. The system of claim 1 wherein the controller is further configured to measure at least two of the plurality of measurable capacitances using a same one of the at least one charge changing circuit.

4. The system of claim 1 wherein a passive network of the at least one passive network includes at least two integrating capacitances, and wherein the two integrating capacitances are coupled to a power supply voltage and provide a conditioned voltage that varies with the threshold of the quantizer.

5. The system of claim 1 wherein each of the plurality of measurable capacitances comprises a capacitance measured between a sensor electrode in a plurality of sensor electrodes and a proximate external object.

6. The system of claim 1 wherein each of the plurality of measurable capacitances comprises a transcapacitance measured between sensor electrodes in a plurality of sensor electrodes.

7. The system of claim 6 wherein at least two of the plurality of measurable capacitances are measured concurrently and where the controller is configured to accumulate charge from each of the at least two of the plurality of measurable capacitances on a corresponding one of at least two integrating capacitances.

8. The system of claim 6 wherein the controller is configured to differentially measure at least two of the plurality of measurable capacitances concurrently by repeatedly applying distinct voltages to the at least two of the plurality of digital outputs coupled to the measurable capacitances.

9. The system of claim 1 further comprising a voltage conditioning circuit coupled to the at least one passive network, the voltage conditioning circuit adapted to provide a variable reference voltage to that varies with a threshold voltage of the quantizer.

10. The system of claim 9 wherein the voltage conditioning circuit includes a voltage divider having a first divider impedance and a second divider impedance, the first divider impedance and the second divider impedance coupled at a divider node, and wherein the voltage conditioning circuit is configured to couple a power supply voltage to the at least one passive network through the voltage divider node.

11. The system of claim 10 wherein the first divider impedance comprises a first divider capacitance and wherein the second divider impedance comprises a second divider capacitance.

12. The system of claim 11 further comprising a resistance coupled to the power supply voltage and the divider node in parallel with the first divider capacitance.

13. The system of claim 10 wherein the first divider impedance comprises a first divider resistance and wherein the second divider impedance comprises a second divider resistance.

14. The system of claim 1 further comprising a guarding electrode coupled to an output of the controller, the guarding electrode proximate each of the plurality of measurable capacitances.

15. The system of claim 14 wherein the guarding electrode is coupled to the controller through a guarding network, and wherein the guarding network comprises a voltage divider.

16. The system of claim 14 wherein the controller is further configured to apply the guard signal by switching the output between a first reference voltage and a second reference voltage.

17. The system of claim 14 wherein the controller is further configured to apply the guard signal by switching the output between a first reference voltage and an open circuit condition.

18. The system of claim 14 further comprising a guarding network coupling the guarding electrode and the output, wherein the guarding network comprises a first guard capacitance and a second guard capacitance substantially greater than the first guard capacitance, and wherein the controller is further configured to apply the guard signal by switching the output between a first reference voltage and an open circuit condition and accumulating charge on the second guard capacitance.

19. The system of claim 14 wherein the output is a digital output of the controller.