Continuous digital offset cancellation

1. A disk drive comprising:

a disk;

a head for writing data to the disk; and

control circuitry coupled to the head, the control circuitry comprising a signal amplifier outputting a first signal comprising an offset voltage, the control circuitry operable to:

determine a state signal based on comparing the first signal to a reference signal;

increase a M-bit digital count value in response to a first state of the state signal;

decrease the M-bit digital count value in response to a second state of the state signal; and

determine a first analog signal based on N bits of the digital count value, wherein the first analog signal adjusts the offset voltage and M>N.

2. The disk drive of claim 1 , wherein the control circuitry comprises a comparator coupled to the signal amplifier and the reference signal to determine the state signal.

3. The disk drive of claim 2 , wherein the comparator outputs the first state when the first signal is greater than the reference signal.

4. The disk drive of claim 1 , wherein the control circuitry comprises an M-bit counter responsive to the state signal to update the M-bit digital count value according to a clock signal received at a clock input.

5. The disk drive of claim 4 , wherein the M-bit counter increases the M-bit digital count value in response to the first state and decreases the M-bit digital count value in response to the second state.

6. The disk drive of claim 5 , wherein the control circuitry comprises an N-bit digital-to-analog converter (DAC) coupled to the M-bit counter.

7. The disk drive of claim 6 , wherein the clock signal comprises a frequency F s and the voltage offset correction is provided from a frequency range of 0 Hz to F s /2 (M−N) Hz.

8. The disk drive of claim 6 , wherein a DAC output voltage step size equals V DAC /2 N wherein V DAC is a voltage range of the N-bit DAC.

9. The disk drive of claim 6 , wherein the M-bit counter outputs N most significant bits (MSB) to the N-bit DAC.

10. The disk drive of claim 1 , wherein the signal amplifier comprises a first input and the control circuitry comprises a combining junction coupled to an input signal, the first analog signal, and the first input.

11. The disk drive of claim 10 , wherein the combining junction subtracts the first analog signal from the input signal.

12. The disk drive of claim 1 , wherein the control circuitry further comprises a shock sensor coupled to the signal amplifier.

13. The disk drive of claim 12 , wherein the control circuitry further comprises a window comparator coupled to the signal amplifier to produce a shock interrupt signal.

14. The disk drive of claim 1 , wherein the control circuitry further comprises a pressure sensor coupled to the signal amplifier.

15. The disk drive of claim 1 , wherein the control circuitry is coupled to a voice coil motor (VCM) to measure a back electro-motive force (EMF).

16. The disk drive of claim 1 , wherein the control circuitry further comprises a rotational accelerometer sensor coupled to the signal amplifier.

17. The disk drive of claim 1 , wherein the offset voltage comprises a DC offset voltage.

18. The disk drive of claim 1 , wherein the control circuitry is further operable to continuously adjust the offset voltage.

19. A method of correcting an offset in a disk drive, comprising:

outputting a first signal comprising an offset voltage from a signal amplifier;

determining a state signal based on comparing the first signal to a reference signal;

increasing a M-bit digital count value in response to a first state of the state signal;

decreasing the M-bit digital count value in response to a second state of the state signal;

determining a first analog signal based on N bits of the digital count value, wherein M>N;

adjusting the offset voltage of the first signal based on the first analog signal; and

controlling a head in response to the adjusted first signal.

20. The method of claim 19 , wherein determining the state signal based on comparing the first signal to the reference signal comprises determining the state signal based on an output of a comparator coupled to the signal amplifier and the reference signal.

21. The method of claim 20 , wherein the comparator outputs the first state when the first signal is greater than the reference signal.

22. The method of claim 19 , wherein the M-bit digital count value is output by an M-bit counter responsive to the state signal and a clock signal received at a clock input.

23. The method of claim 22 , wherein the M-bit counter increases the M-bit digital count value in response to the first state and decreases the M-bit digital count value in response to the second state.

24. The method of claim 23 , wherein determining the first analog signal based on N bits of the digital count value comprises inputting N bits of the digital count value into an N-bit digital-to-analog converter (DAC).

25. The method of claim 24 , wherein the clock signal comprises a frequency F s and the voltage offset correction is provided from a frequency range of 0 Hz to F s /2 (M−N) Hz.

26. The method of claim 24 , wherein a DAC output voltage step size equals V DAC /2 N wherein V DAC is a voltage range of the N-bit DAC.

27. The method of claim 24 , wherein the M-bit counter outputs N most significant bits (MSB) to the N-bit DAC.

28. The method of claim 19 , further comprises combining an input signal and the first analog signal to provide to a first input to the signal amplifier.

29. The method of claim 28 , further comprises subtracting the first analog signal from the input signal.

30. The method of claim 19 , further comprises inputting a shock sensor signal to the signal amplifier.

31. The method of claim 30 , further comprises determining a shock interrupt signal based on a window comparator and the first signal.

32. The method of claim 19 , wherein the offset voltage comprises a DC offset voltage.

33. The method of claim 19 , further comprising continuously adjusting the offset voltage.