Background calibration technique for pipelined A/D converters using simplified histogram-based testing
View Patent ↗A method and apparatus are provided for calibrating a multi-stage A/D converter, such as a pipelined A/D converter. The method and apparatus are based on estimating the bounds of histograms of codes from various stages in the A/D converter. Known approaches were effective in calibrating A/D converters but during bound estimation suffered from lock-up conditions from which it could not recover. Embodiments of the present invention describe two mechanisms for recovering from lock-up conditions and a mechanism for fast locking. If neither a gross lock-up condition nor a fine lock-up condition is detected, the estimated bound is modified based on a comparison of a current digital residue with a fast lock value and a bound window. A discontinuity in the transfer characteristic of the A/D converter can then be removed based on the estimated bound.
1. A calibration method for a multi-stage analog to digital (A/D) converter, each stage having an analog input, a digital output, an analog residue and a digital residue, a histogram of digital residue codes being used to correct a transfer characteristic of the stage for a given digital output, the method comprising:
estimating a bound for a current histogram associated with a current digital output code, the estimated bound being used to estimate a discontinuity in the transfer characteristic;
in response to a determination that a current digital residue occurs in the current histogram:
comparing the estimated bound with a gross lock-up detection value and a fine lock-up detection characteristics;
detecting whether a gross lock-up condition exists based on the comparison of the estimated bound with the gross lock-up detection value;
restarting calibration in response to detection of the gross lock-up condition;
in response to detection that there is no gross lock-up condition, detecting whether a fine lock-up condition exists by determining whether the current digital residue satisfies fine lock-up detection characteristics;
in response to detection that there is no fine lock-up condition, modifying the estimated bound based on a comparison of the current digital residue with a fast lock value and a bound window; and
removing the discontinuity in the transfer characteristic of the A/D converter based on the estimated bound.
2. The method of claim 1 wherein modifying the estimated bound comprises setting the estimated bound to equal the current digital residue in response to detection that the current digital residue exceeds the fast lock value.
3. The method of claim 1 wherein modifying the estimated bound comprises setting the estimated bound to equal a previous digital residue in response to detection that the current digital residue has exceeded the fast lock value for a fast lock detection period.
4. The method of claim 1 wherein detecting whether the fine lock-up condition exists by determining whether the current digital residue satisfies fine lock-up detection characteristics comprises:
computing a count of a number of times that the current or previous digital residues for the current digital output code are detected in the current histogram and in a second histogram of the stage for a second digital output adjacent to the current digital output;
determining whether the current or previous digital residues for the current digital output code are in the bound window; and
detecting the fine lock-up condition when the computed count exceeds a fine lock-up detection threshold and there is no current or previous digital residue for the current digital output code in the bound window.
5. The method of claim 1 wherein comparing the estimated bound with the gross lock-up detection value and the fine lock-up detection characteristics comprises comparing the gross lock-up detection value and the fine lock-up detection characteristics with a difference between the estimated bound and an opposing estimated bound.
6. The method of claim 1 wherein the estimated bound comprises a difference between an upper histogram bound estimate and a lower histogram bound estimate.
7. The method of claim 1 wherein the gross lock-up detection value is calculated based on expected gain error.
8. The method of claim 1 wherein the gross lock-up detection value is calculated based on maximum expected mismatch in analog components in the analog to digital converter.
9. The method of claim 1 wherein the analog to digital converter comprises a pipelined analog to digital converter.
10. The method of claim 1 wherein the steps are repeated for each of a plurality of digital output values.
11. The method of claim 1 wherein the steps are repeated for each stage requiring calibration.
12. A computer-readable medium storing statements and instructions which, when executed, cause a processor to perform a calibration method for a multi-stage analog to digital (A/D) converter, each stage having an analog input, a digital output, an analog residue and a digital residue, a histogram of digital residue codes being used to correct a transfer characteristic of the stage for a given digital output, the method comprising:
estimating a bound for a current histogram associated with a current digital output code, the estimated bound being used to estimate a discontinuity in the transfer characteristic;
in response to a determination that a current digital residue occurs in the current histogram:
comparing the estimated bound with a gross lock-up detection value and a fine lock-up detection characteristics;
detecting whether a gross lock-up condition exists based on the comparison of the estimated bound with the gross lock-up detection value;
restarting calibration in response to detection of the gross lock-up condition;
in response to detection that there is no gross lock-up condition, detecting whether a fine lock-up condition exists by determining whether the current digital residue satisfies fine lock-up detection characteristics;
in response to detection of that there is no fine lock-up condition, modifying the estimated bound based on a comparison of the current digital residue with a fast lock value and a bound window; and
removing the discontinuity in the transfer characteristic of the A/D converter based on the estimated bound.
13. The computer readable medium of claim 12 wherein, in the method, modifying the estimated bound comprises setting the estimated bound to equal the current digital residue in response to detection that the current digital residue exceeds the fast lock value.
14. The computer readable medium of claim 12 wherein, in the method, modifying the estimated bound comprises setting the estimated bound to equal a previous digital residue in response to detection that the current digital residue has exceeded the fast lock value for a fast lock detection period.
15. The computer readable medium of claim 12 wherein, in the method, detecting whether the fine lock-up condition exists by determining whether the current digital residue satisfies fine lock-up detection characteristics comprises:
computing a count of a number of times that the current or previous digital residues for the current digital output code are detected in the current histogram and in a second histogram of the stage for a second digital output adjacent to the current digital output;
determining whether the current or previous digital residues for the current digital output code are in the bound window;
detecting the fine lock-up condition when the computed count exceeds a fine lock-up detection threshold and there is no current or previous digital residue for the current digital output code in the bound window.
16. The computer readable medium of claim 12 wherein, in the method, comparing the estimated bound with the gross lock-up detection value and the fine lock-up detection characteristics comprises comparing the gross lock-up detection value and the fine lock-up detection characteristics with a difference between the estimated bound and an opposing estimated bound.
17. The computer readable medium of claim 12 wherein, in the method, the estimated bound comprises a difference between an upper histogram bound estimate and a lower histogram bound estimate.
18. The computer readable medium of claim 12 wherein, in the method, the gross lock-up detection value is calculated based on expected gain error.
19. The computer readable medium of claim 12 wherein, in the method, wherein the gross lock-up detection value is calculated based on maximum expected mismatch in analog components in the analog to digital converter.
20. The computer readable medium of claim 12 wherein, in the method, the analog to digital converter comprises a pipelined analog to digital converter.
21. The computer readable medium of claim 12 wherein, in the method, the current bound estimate is incremented or decremented by a bound update step size.
22. The computer readable medium of claim 12 wherein the method steps are repeated for each of a plurality of digital output values.
23. The computer readable medium of claim 12 wherein the method steps are repeated for each stage requiring calibration.