METHODS AND SYSTEMS FOR DETECTING A CAPACITANCE USING SWITCHED CHARGE TRANSFER TECHNIQUES

• US 20080042660A1
• Filed: 10/26/2007
• Published: 02/21/2008
• Est. Priority Date: 06/03/2005
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: HARGREAVES, Kirk, ELY, David, ROUTLEY, Paul, REYNOLDS, Joseph, HAINES, Julian
• Granted Patent: US 7,521,941 B2
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/0383 Signal control means within the pointing device
• 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 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03J 9/00 Remote-control of tuned circuits; Combined remote-control of tuning and other functions, e.g. brightness, amplification mechanical remote-control arrangements H03J1/00 ; using a microprocessor H03J1/0025; constructional details of remote control switching devices H01H9/0235
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03K 2217/960775 Emitter-receiver or "fringe" type detection, i.e. one or more field emitting electrodes and corresponding one or more receiving electrodes

Abstract

Methods, systems and devices are described for detecting a measurable capacitance using charge transfer techniques. According to various embodiments, a charge transfer process is performed for two or more times. During the charge transfer process, a pre-determined voltage is applied to the measurable capacitance, and the measurable capacitance is then allowed to share charge with a filter capacitance through a passive impedance that remains coupled to both the measurable capacitance and to the filter capacitance throughout the charge transfer process. The value of the measurable capacitance can then be determined as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed. Such a detection scheme may be readily implemented using conventional components, and can be particularly useful in sensing the position of a finger, stylus or other object with respect to an input sensor.

Claims

1. A circuit for measuring a measurable capacitance at a first electrical node, the circuit comprising:

a passive impedance statically coupled between the first electrical node and a second electrical node;

a filter capacitance coupled between the second electrical node and a third electrical node; and

a controller coupled to the first electrical node, wherein the controller is configured to perform a charge transfer process for a number of times greater than one, wherein the charge transfer process comprises applying a predetermined voltage to the first electrical node for a period of time and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the measurable capacitance as a function of a voltage on the filter capacitance and the number of times.

2. The circuit of claim 1 further comprising a second filter capacitance coupled between the second electrical node and a fourth electrical node.

3. The circuit of claim 2 wherein the third electrical node is coupled to a first power supply voltage, and wherein the fourth electrical node is coupled to a second power supply voltage different from the first power supply voltage.

4. The circuit of claim 1 wherein the third electrical node is connected to a reference voltage.

5. The circuit of claim 1 wherein the third electrical node is coupled to the controller.

6. The circuit of claim 5 further comprising a second measurable capacitance coupled to the third electrical node.

7. The circuit of claim 1 wherein the second electrical node is coupled to the controller and the third electrical node is coupled to the controller.

8. The circuit of claim 7 further comprising:

a fourth electrical node coupled to the controller;

a second passive impedance coupled to the third electrical node and to the fourth electrical node; and

a second measurable capacitance coupled to the fourth electrical node.

9. The circuit of claim 1 further comprising an impedance coupled to the third electrical node and a reference voltage, wherein the impedance is configured to add at least a portion of the reference voltage to the voltage on the filter capacitance.

10. The circuit of claim 1 further comprising an impedance divider selected from the group consisting of a capacitive divider network and a resistive divider network, wherein the impedance divider is coupled to the third electrical node and a reference voltage, and wherein the impedance is configured to add at least a portion of the reference voltage to the voltage on the filter capacitance.

11. The circuit of claim 1 wherein the passive impedance a resistor.

12. The circuit of claim 1 wherein the controller is further configured to compare the voltage at the first electrical node to a reference voltage to ascertain the voltage on the filter capacitance.

13. The circuit of claim 12 wherein the controller is further configured to compare the voltage at the first electrical node with a second reference voltage to ascertain the voltage on the filter capacitance.

14. The circuit of claim 13 wherein the first reference voltage and the second reference voltage are provided by a digital input of the controller.

15. The circuit of claim 1 wherein the controller is further configured to measure the voltage at the first electrical node using one of a digital input and a multi-bit analog-to-digital converter to ascertain the voltage on the filter capacitance.

16. The circuit of claim 1 wherein the controller is further configured to set the voltage on the filter capacitance to a known value to reset the filter capacitance.

17. The circuit of claim 16 wherein the controller is further configured to couple the filter capacitance to a reset voltage through an impedance for a pre-determined length of time to reset the filter capacitance.

18. The circuit of claim 1, wherein the controller is further connected to the second electrical node, and wherein the controller is further configured to compare the voltage at the second electrical node with a reference voltage to ascertain the voltage on the filter capacitance.

19. A device for measuring a measurable capacitance, the device comprising:

a filter capacitance means for storing charge;

means for applying a pre-determined voltage to the measurable capacitance;

passive impedance means statically coupled to both the measurable capacitance and the filter capacitance means for allowing the measurable capacitance to share charge with the filter capacitance means; and

means for determining a value of the measurable capacitance as a function of a representation of a charge on the filter capacitance means.

20. A proximity sensor comprising:

a sensor electrode having a measurable capacitance;

a switch coupled to the measurable capacitance;

a passive network coupled to the measurable capacitance and the switch, the passive network comprising a passive impedance and a filter capacitance, wherein the passive impedance statically couples the measurable capacitance to the filter capacitance; and

a controller coupled to the switch, wherein the controller is configured to perform a charge transfer process for a number of times greater than one, wherein the charge transfer process comprises applying a pre-determined voltage to the measurable capacitance using the switch and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the measurable capacitance as a function of a representation of a charge of the filter capacitance and the number of times.

21. The proximity sensor of claim 20 further comprising:

a second sensor electrode having a second measurable capacitance; and

a second switch coupled to the second measurable capacitance,

a second passive network,

wherein the second passive network is coupled to the second measurable capacitance and the second switch, wherein the second passive network further comprises a second passive impedance and a second filter capacitance, and wherein the second passive impedance statically couples the second measurable capacitance to the second filter capacitance; and

wherein the controller is coupled to the second switch and is further configured to perform a second charge transfer process for a second number of times greater than one, wherein the second charge transfer process comprises applying a second pre-determined voltage to the second measurable capacitance using the second switch and allowing the second measurable capacitance to share charge with the second filter capacitance through the second passive impedance, and wherein the controller is further configured to determine a value of the second measurable capacitance as a function of a representation of a charge of the second filter capacitance and the second number of times.

22. The proximity sensor of claim 20 further comprising:

a second sensor electrode having a second measurable capacitance; and

a second switch coupled to the second measurable capacitance,

wherein the passive network is coupled to the second measurable capacitance and the second switch, and wherein the passive impedance statically couples the second measurable capacitance to the filter capacitance, and

wherein the controller is coupled to the second switch and is further configured to perform a second charge transfer process for a second number of times greater than one, wherein the second charge transfer process comprises applying a second pre-determined voltage to the second measurable capacitance using the second switch and allowing the second measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the second measurable capacitance as a function of a second representation of a second charge of the filter capacitance and the second number of times.

23. The proximity sensor of claim 20 further comprising:

a second sensor electrode having a second measurable capacitance;

which is not connected directly to the passive network.

a multiplexer coupled to the first measurable capacitance and the second measurable capacitance and the passive network,

wherein the controller is coupled to the multiplexer and is further configured to multiplex the passive network at least between the first measurable capacitance and the second measurable capacitance using the multiplexer, and

wherein the controller is coupled to the switch and is further configured to perform a second charge transfer process for a second number of times greater than one, wherein the second charge transfer process comprises applying a second pre-determined voltage to the second measurable capacitance through the multiplexer and allowing the second measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the second measurable capacitance as a function of a second representation of a second charge on the filter capacitance and the second number of.

24. The proximity sensor of claim 20 wherein the switch comprises a digital output of a controller.

25. The proximity sensor of claim 20 wherein the controller is further configured to provide a guard signal configured to guard the measurable capacitance during at least a portion of the charge transfer process.