Touch controller with improved analog front end

• US 8,031,094 B2
• Filed: 09/11/2009
• Issued: 10/04/2011
• Est. Priority Date: 09/11/2009
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Hotelling, Steven Porter, Krah, Christoph Horst
• Primary Examiner(s): WILLIAMS, HOWARD L
• Application Number:
  US12/558,374
• Publication Number:
  US 20110063154A1
• Time in Patent Office: 753 Days
• Field of Search: 341/143, 341/139, 345/173, 345/174
• US Class Current: 341/143
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes
• CPC Class Codes: H03M 3/494 Sampling or signal conditioning arrangements specially adapted for delta-sigma type analogue/digital conversion systems

Abstract

A controller for a touch sensor includes a transimpedance amplifier, and a feedback resistor coupled to an input of the transimpedance amplifier and to an output of the transimpedance amplifier. At least one multiplexor may be coupled to the input of the transimpedance amplifier and configured to multiplex a plurality of analog inputs to one dedicated channel. The controller may further include a bandpass filter coupled to the output of the transimpedance amplifier. The output of the bandpass filter may be input to an anti-aliasing filter, which feeds into an analog to digital converter. Alternatively, the output of the bandpass filter may be input to a sigma-delta analog to digital converter.

Claims

1. An analog front end for receiving a sense signal from a touch sensor, comprising:

a transimpedance amplifier;

a feedback resistor coupled to an input of the transimpedance amplifier and to an output of the transimpedance amplifier; and

a bandpass filter coupled to the output of the transimpedance amplifier;

wherein a gain of the bandpass filter is adjustable to increase noise headroom.

2. The analog front end of claim 1, wherein the bandpass filter has a programmable center frequency.

3. The analog front end of claim 1, further comprising:

an analog to digital converter coupled to the output of the bandpass filter.

4. The analog front end of claim 3, wherein the analog to digital converter is a sigma-delta analog to digital converter.

5. The analog front end of claim 3, further comprising:

an anti-aliasing filter coupled between the output of the bandpass filter and the analog to digital converter.

6. The analog front end of claim 5, wherein a cut-off frequency of the anti-aliasing filter can be adjusted based on a selected center frequency of the bandpass filter as to optimize bandpass filter roll-off.

7. The analog front end of claim 1, further comprising:

at least one multiplexor coupled to the input of the transimpedance amplifier configured to multiplex a plurality of sense signals to one analog front end.

8. The analog front end of claim 1, wherein the touch sensor is part of a computing system.

9. The analog front end of claim 1, wherein the bandpass filter has a passband gain of at least 6 dB.

10. A method for receiving a sense signal from a touch sensor, comprising:

amplifying an incoming signal using a transimpedance amplifier having a feedback resistor coupled to an input of the transimpedance amplifier and to an output of the transimpedance amplifier; and

filtering an output of the transimpedance amplifier using a bandpass filter having a programmable center frequency; and

adjusting a gain of the bandpass filter to increase noise headroom.

11. The method of claim 10, further comprising:

digitizing an output of the bandpass filter to enable digital demodulation.

12. The method of claim 11, wherein the analog to digital converter is a sigma-delta analog to digital converter.

13. The method of claim 10, further comprising:

attenuating noise by performing anti-aliasing filtering of an output of the bandpass filter.

14. The method of claim 13, wherein a cut-off frequency of the anti-aliasing filtering can be adjusted based on the programmable center frequency so as to optimize bandpass filter roll-off.

15. The method of claim 10, further comprising:

multiplexing a plurality of analog inputs to one dedicated channel to be input to the transimpedance amplifier.

16. The method of claim 10, wherein the touch sensor controller is part of a computing system.

17. The method of claim 10, wherein the bandpass filter has a passband gain of at least 6 dB.

18. A controller for a touch sensor, comprising:

a pre-amplifier;

a feedback resistor coupled to an input of the pre-amplifier, an output of the pre-amplifier and a virtual ground;

at least one capacitor coupled to the input of the pre-amplifier and the virtual ground;

a sigma-delta analog to digital converter coupled to the output of the pre-amplifier; and

a feedback circuit coupled between an output of the sigma-delta analog to digital converter and the input of the pre-amplifier to provide variable gain adjustment and bandpass filter response.

19. The controller of claim 18, wherein the sigma-delta analog to digital converter is of a second order or higher.

20. The controller of claim 18, wherein two capacitors are coupled to the input of the pre-amplifier, with at least one capacitor coupled to virtual ground and at least one capacitor has a variable capacitance.

21. A controller for a touch sensor, comprising:

a pre-amplifier, the input of which is coupled to a virtual ground; and

a sigma-delta analog to digital converter coupled to the output of the pre-amplifier, wherein the sigma-delta analog to digital converter has bandpass filter response, with one or a plurality of feedback paths to the input of the pre-amplifier, which is held at virtual ground.

22. The controller of claim 21, wherein the bandpass filter response is realized with a resonator.

23. The controller of claim 21, wherein the sigma-delta analog to digital converter is of a second order or higher.

24. The controller of claim 21, further comprising:

a digital demodulator coupled to the output of the sigma-delta analog to digital converter.

25. An analog front end for receiving a sense signal from a touch sensor, comprising:

a biquad bandpass filter including a virtual ground input for rejecting stray capacitance from the touch sensor; and

an analog to digital converter (ADC) coupled to the biquad bandpass filter.

26. The analog front end of claim 25, where the biquad bandpass filter has at least one of a programmable center frequency, gain and Q.

27. The analog front end of claim 25, where the ADC is a sigma-delta convertor.

28. The analog front end of claim 25, where the biquad bandpass filter is a tow-thomas biquad bandpass filter.