SENSE AMPLIFIER STABILIZATION WITH SWITCHED SERIES RESISTOR

• US 20160216836A1
• Filed: 01/27/2015
• Published: 07/28/2016
• Est. Priority Date: 01/27/2015
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: YAO, Weijun, YAO, Wei Hsin, LI, Yingxuan
• Granted Patent: US 9,817,513 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G06F 3/04164 Connections between sensors and controllers, e.g. routing lines between electrodes and connection pads
• CPC Class Codes: G06F 3/041661 using detection at multiple resolutions, e.g. coarse and fine scanning; using detection within a limited area, e.g. object tracking window
• CPC Class Codes: G06F 3/0443 using a single layer of sensing electrodes

Abstract

A sense amplifier can be designed with a series variable resistance circuit to dynamically adjust an input resistance in series with the sense amplifier for stability during the different operating modes. In some examples, a switchable resistor can be switched into the input path of the sense amplifier for stability when required by increased capacitive loading of the sense electrodes, and can be switched out of the input path of the sense amplifier when unnecessary for stability. In some examples, an adjustable resistor can be coupled to and in series with the input of the sense amplifier. The resistance of the adjustable resistor can be tuned to stabilize the sense amplifier during operating modes having increased capacitive loading from the sense electrodes and can be minimized when additional input resistance is unnecessary for stability.

Claims

1. A touch sensing apparatus comprising:

a sense channel configured to receive input from one or more electrodes, the sense channel comprising a sense amplifier and a variable resistance circuit, the variable resistance circuit coupled to an input of the sense amplifier and in series with the sense amplifier; and

logic configured to;

couple the sense channel to the one or more electrodes; and

adjust an effective resistance of the variable resistance circuit to satisfy one or more stability conditions for the sense amplifier.

2. The touch sensing apparatus of claim 1, the logic further configured to:

in a first sensing mode, couple the sense channel to a plurality of electrodes and adjust the effective resistance of the variable resistance circuit to satisfy the one or more stability conditions for the sense amplifier; and

in a second sensing mode, couple the sense channel to one electrode and minimize the effective resistance of the variable resistance circuit to satisfy the one or more stability conditions for the sense amplifier.

3. The touch sensing apparatus of claim 1, wherein satisfying the one or more stability conditions comprises limiting a phase shift of a transfer function representing a circuit comprising the sense amplifier and a capacitive loading of the one or more coupled electrodes to within a threshold phase margin of 180 degrees of phase shift for a unity gain bandwidth of the sense amplifier.

4. The touch sensing apparatus of claim 1, wherein adjusting the effective resistance of the variable resistance circuit to satisfy the one or more stability conditions for the sense amplifier comprises increasing the effective resistance of the variable resistance circuit to introduce a zero in a transfer function of a circuit including the sense amplifier and the one or more electrodes to stabilize the sense amplifier due to effects of increased input capacitance from the one or more electrodes on a pole of the transfer function.

5. The touch sensing apparatus of claim 1, wherein the effective resistance of the variable resistance circuit can be adjusted based on a parameter from a look-up table based on a number of the one or more electrodes or an estimated input capacitance from the one or more electrodes.

6. The touch sensing apparatus of claim 1, wherein the variable resistance circuit comprises a switch and a resistor, and dynamically adjusting the variable resistance circuit comprises closing the switch to bypass the resistor.

7. The touch sensing apparatus of claim 1, wherein the variable resistance circuit comprises a plurality of resistors and a plurality switches, and dynamically adjusting the effective resistance of the variable resistance circuit comprises configuring the plurality of switches to generate a desired equivalent resistance.

8. The touch sensing apparatus of claim 1, wherein the variable resistance circuit comprises a variable resistor.

9. The touch sensing apparatus of claim 1, wherein the effective resistance of the variable resistance circuit is zero when a number of electrodes coupled to the sense channel is below a threshold number of electrodes and the effective resistance of the variable resistance circuit is adjusted above zero and corresponding to the number of electrodes coupled to the sense channel when the number of electrodes coupled to the sense channel meets or exceeds the threshold number of electrodes.

10. The touch sensing apparatus of claim 1, wherein the effective resistance of the variable resistance circuit is zero when a number of electrodes coupled to the sense channel is one and the effective resistance of the variable resistance circuit is adjusted above zero and corresponding to the number of electrodes coupled to the sense channel when the number of electrodes coupled to the sense channel is greater than one.

11. The touch sensing apparatus of claim 1, incorporated into a touch system comprising a touch sensor panel, wherein the touch sensor panel includes a plurality of electrodes, each electrode representing a unique location of the touch sensor panel at which touch or proximity events can be sensed, and each electrode being electrically isolated from other electrodes in the touch sensor panel.

12. The touch sensing apparatus of claim 11, wherein the sense channel is configured to sense a self-capacitance of one or more of the plurality of electrodes of the touch sensor panel.

13. A method for scanning a touch sensor panel, the method comprising:

configuring a plurality of sense channels for a coarse scan operation, wherein each of the sense channels configured for the coarse scan operation includes an adjustable resistance in series with an input path of a sense amplifier;

performing the coarse scan operation;

configuring the plurality of sense channels for a fine scan operation, wherein the adjustable resistance in series with the input path of the sense amplifier is reduced for each of the sense channels configured for the fine scan operation; and

performing the fine scan operation.

14. The method of claim 13, wherein the touch sensor panel includes electrodes, each electrode representing a unique location of the touch sensor panel at which touch or proximity events can be sensed, and each electrode being electrically isolated from other electrodes in the touch sensor panel.

15. The method of claim 14, wherein configuring the plurality of sense channels for the coarse scan operation further comprises coupling each of the plurality of sense channels to a plurality of the electrodes and adjusting the adjustable resistance in series with the input path of the sense amplifier in each of the plurality of sense channels to satisfy one or more stability conditions for the sense amplifier;

and

wherein configuring the plurality of sense channels for the fine scan operation further comprises coupling the plurality of sense channels to one of the electrodes and adjusting the adjustable resistance in series with the input path of the sense amplifier in each of the plurality of sense channels to satisfy the one or more stability conditions for the sense amplifier.

16. The method of claim 15, wherein satisfying the one or more stability conditions comprises limiting a phase shift of a transfer function representing a circuit comprising the sense amplifier and capacitive loading of one or more electrodes to within a threshold phase margin of 180 degrees of phase shift for a unity gain bandwidth of the sense amplifier.

17. The method of claim 15, wherein adjusting the adjustable resistance to satisfy the one or more stability conditions for the sense amplifier comprises introducing a zero in a transfer function of a circuit including the sense amplifier and one or more electrodes to stabilize the sense amplifier due to effects of increased input capacitance from the one or more electrodes on a pole of the transfer function.

18. The method of claim 15, wherein the adjustable resistance can be adjusted based on a parameter from a look-up table based on a number of electrodes coupled to the sense channel or an estimated input capacitance from one or more electrodes.

19. The method of claim 15, wherein the adjustable resistance is zero when a number of electrodes coupled to the sense channel is below a threshold number of electrodes and the adjustable resistance is adjusted above zero and corresponding to the number of electrodes coupled to the sense channel when the number of electrodes coupled to the sense channel meets or exceeds the threshold number of electrodes.

20. The method claim 15, wherein the adjustable resistance is zero when a number of electrodes coupled to the sense channel is one and the adjustable resistance is adjusted above zero and corresponding to the number of electrodes coupled to the sense channel when the number of electrodes coupled to the sense channel is greater than one.

21. A non-transitory computer readable storage medium containing instructions that, when executed by a processor, can perform a method for scanning a touch sensor panel, the method comprising:

configuring a plurality of sense channels for a coarse scan operation, wherein each of the sense channels configured for the coarse scan operation includes an adjustable resistance in series with an input path of a sense amplifier;

performing the coarse scan operation;

configuring the plurality of sense channels for a fine scan operation, wherein the adjustable resistance in series with the input path of the sense amplifier is reduced for each of the sense channels configured for the fine scan operation; and

performing the fine scan operation.

22. The non-transitory computer readable storage medium of claim 21, wherein the touch sensor panel includes electrodes, each electrode representing a unique location of the touch sensor panel at which touch or proximity events can be sensed, and each electrode being electrically isolated from other electrodes in the touch sensor panel.

23. The non-transitory computer readable storage medium of claim 22, wherein configuring the plurality of sense channels for the coarse scan operation further comprises coupling each of the plurality of sense channels to a plurality of the electrodes and adjusting the adjustable resistance in series with the input path of the sense amplifier in each of the plurality of sense channels to satisfy one or more stability conditions for the sense amplifier;

and

wherein configuring the plurality of sense channels for the fine scan operation further comprises coupling the plurality of sense channels to one of the electrodes and adjusting the adjustable resistance in series with the input path of the sense amplifier in each of the plurality of sense channels to satisfy the one or more stability conditions for the sense amplifier.

24. The non-transitory computer readable storage medium of claim 23, wherein satisfying the one or more stability conditions comprises limiting a phase shift of a transfer function representing a circuit comprising the sense amplifier and capacitive loading of one or more electrodes to within a threshold phase margin of 180 degrees of phase shift for a unity gain bandwidth of the sense amplifier.

25. The non-transitory computer readable storage medium of claim 23, wherein adjusting the adjustable resistance to satisfy the one or more stability conditions for the sense amplifier comprises introducing a zero in a transfer function of a circuit including the sense amplifier and one or more electrodes to stabilize the sense amplifier due to effects of increased input capacitance from the one or more electrodes on a pole of the transfer function.

26. A touch sensing apparatus comprising:

a sense channel configured to receive input from one or more electrodes, the sense channel comprising a sense amplifier and means for adjusting an input resistance coupled to an input of the sense amplifier and in series with the sense amplifier;

means for coupling the sense channel to the one or more electrodes; and

means for adjusting the input resistance to satisfy one or more stability conditions for the sense amplifier.