Stylus and stylus circuitry for capacitive touch screens

1. A circuit for a capacitive touch screen stylus, the circuit comprising:

a. a conductive tip for providing capacitive coupling with a capacitive touch screen;

b. an inverting charge integrator connected to the conductive tip for providing an output signal proportional to a charge induced at the conductive tip;

c. an inverting amplifier for generating an amplified signal proportional to the signal output from the inverting charge integrator;

d. a conductive contact for providing electrical or capacitive coupling between the amplified signal and an exterior of the stylus; and

e. a power supply circuit for powering the inverting charge integrator and the inverting amplifier.

2. The circuit of claim 1 wherein a positive transition in potential at the tip results in a positive transition in voltage at the conductive contact.

3. The circuit of claim 2 wherein the batteries are rechargeable.

4. The circuit of claim 1 wherein a negative transition in potential at the tip results in a negative transition in voltage at the conductive contact.

5. The circuit of claim 1 wherein the inverting charge integrator and the inverting amplifier comprise one or more operational amplifiers.

6. The circuit of claim 1 additionally comprising shielding for isolating at least the connection between the tip and the inverting charge integrator.

7. The circuit of claim 6 wherein the shielding is connected to a ground of the stylus circuit.

8. The circuit of claim 6 wherein the shielding comprises a conductive enclosure having a circular cross-section and extending along an axis of at least a portion of the stylus.

9. The circuit of claim 6 wherein the shielding comprises a conductive wrapping around at least a portion of the stylus circuit.

10. The circuit of claim 6 wherein the shielding comprises a substantially continuous conducting layer of a circuit board for the stylus circuit.

11. The circuit of claim 6 wherein the shielding comprises one or more conducting planes above and/or below a plane of the stylus circuit.

12. The circuit of claim 6 wherein the shielding comprises one or more conducting planes in a plane of the stylus circuit.

13. The circuit of claim 1 wherein the conductive contact is connected or coupled to an external surface of a stylus.

14. The circuit of claim 1 wherein the inverting charge integrator and the inverting amplifier are combined.

15. The circuit of claim 1 wherein the power supply circuit comprises one or more batteries.

16. The circuit of claim 1 wherein the power supply circuit additionally comprises a DC-to-DC converter for increasing voltage supplied to the inverting charge integrator and the inverting amplifier.

17. The circuit of claim 16 wherein an input ground and an output ground of the DC-to-DC converter are common.

18. The circuit of claim 16 wherein the DC-to-DC converter includes regulator circuitry utilizing feedback for regulating an output voltage and wherein an optical isolator communicates a level of the output voltage to a feedback input to the convertor.

19. The circuit of claim 1 wherein the conductive tip is metallic.

20. The circuit of claim 1 wherein the conductive tip comprises a conductive polymer.

21. The circuit of claim 1 wherein the conductive tip has a cross-section projected onto the touch screen of less than 3.5 millimeters.

22. The circuit of claim 1 wherein the conductive tip includes a ball point.

23. A circuit for a capacitive touch screen stylus, the circuit comprising:

a. a conductive tip for providing capacitive coupling with a capacitive touch screen;

b. an inverting transimpedance amplifier connected to the conductive tip for generating an output signal proportional to a current induced at the conductive tip;

c. an inverting integrator for providing an output voltage proportional to an integrated output signal of the inverting transimpedance amplifier;

d. a conductive contact for providing electrical or capacitive coupling between the output voltage from the inverting integrator to an exterior portion of the stylus; and

e. a power supply circuit for powering the inverting transimpedance amplifier first and the inverting integrator.

24. The circuit of claim 23 wherein the conductive tip has a cross-section projected onto the touch screen of less than 3.5 millimeters.

25. The circuit of claim 23 wherein the conductive tip includes a ball point.

26. A stylus for a capacitive touch screen, the stylus comprising:

a. an elongated barrel for enclosing a stylus circuit comprising:

i. an integrator circuit connected to a conductive tip of the stylus for generating an output signal proportional to a charge induced at the conductive tip;

ii. an amplifier circuit for providing an output voltage proportional to the output signal from the integrator circuit; and

iii. a conductive contact for providing electrical or capacitive coupling between the output voltage from the amplifier circuit and a surface of the stylus.

27. The circuit of claim 26 wherein the conductive tip has a cross-section projected onto the touch screen of less than 3.5 millimeters.

28. The circuit of claim 26 wherein the conductive tip includes a ball point.

29. A stylus for a capacitive touch screen, the stylus comprising:

a. an elongated barrel for enclosing a stylus circuit comprising:

i. an amplifier circuit for providing an output signal proportional to the current induced at a conductive tip of the stylus;

ii. an integrator circuit for generating an output voltage proportional to the output signal from the amplifier circuit; and

iii. a conductive contact for providing electrical or capacitive coupling between the output voltage from the integrator circuit and a surface of the stylus.

30. The circuit of claim 29 wherein the conductive tip has a cross-section projected onto the touch screen of less than 3.5 millimeters.

31. The circuit of claim 29 wherein the conductive tip includes a ball point.

32. A capacitive touch screen stylus comprising:

a. an amplifier circuit for providing an output signal proportional to the current induced at a conductive tip of the stylus;

b. an integrator circuit for generating an output voltage proportional to the output signal from the amplifier circuit; and

c. a conductive contact for providing electrical or capacitive coupling between the output voltage from the integrator circuit and a surface of the stylus.

33. The stylus of claim 32 wherein the conductive tip has a cross-section projected onto the touch screen of less than 3.5 millimeters.

34. The stylus of claim 32 wherein the conductive tip includes a ball point.

35. A method for charging a tip of a stylus for a capacitive touch screen, the method comprising:

a. determining an amount of charge induced at a stylus tip by a positive or negative going drive line transition in a capacitive touch screen; and

b. outputting to an exterior of the stylus a positive or negative going voltage change that is proportional to the determined amount of charge wherein a positive going drive line transition results in a positive going output voltage change, and a negative going drive line transition results in a negative going output voltage change.

36. A stylus for a capacitive touch screen comprising:

a. circuitry for determining an amount of charge induced at a stylus tip by a positive or negative going drive line transition in a capacitive touch screen; and

b. circuitry for outputting to an exterior of the stylus a positive or negative going voltage change that is proportional to the determined amount of charge wherein a positive going drive line transition results in a positive going output voltage change, and a negative going drive line transition results in a negative going output voltage change.