Correcting charge transfer inefficiency in a charge coupled analog to digital converter

Abstract

A technique for correcting charge transfer inefficiencies in a Charge Coupled Device (CCD). The basic approach is to estimate the charge entering at a given stage in a CCD pipeline, and to then determine an estimate of the error introduced by the accumulated leftover charge that will be present at a second point, farther down the pipeline. The error is then corrected by injecting a correcting charge at a third point, farther still down the CCD pipeline. The invention is used, in one embodiment, to correct the output of a charge to digital converter, although principals of the invention may be used for other types of circuits.

1. A method comprising:
 (a) estimating an amount of charge entering at a first selected stage in a multiple stage charge transfer pipeline; 
 (b) determining an estimate of an accumulative error introduced at a second selected stage, the second selected stage further along the charge pipeline than the first selected stage; and 
 (c) injecting a correction charge in the pipeline to adjust for the accumulative error, and 
 wherein the step of (b) determining an estimate of the accumulative error further comprises estimating the accumulative error as a function of a per stage transfer efficiency error and the number of stages between the first selected stage and the second selected stage. 
 
     
     
       2. A method as in  claim 1  wherein the per stage transfer efficiency is estimated as a constant percentage of a corresponding stored charge. 
     
     
       3. A method as in  claim 1  wherein the correction charge is injected in the pipeline at a third selected stage which is further along the pipeline than the second selected stage. 
     
     
       4. A method as in  claim 1  wherein the multiple stage charge transfer device is a successive approximation charge to digital converter. 
     
     
       5. A method as in  claim 4  wherein the charge to digital converter is a differential type charge to digital converter that has a positive “p” side of the pipeline and a minus “m” side of the pipeline, and wherein each side of the pipeline is corrected for accumulative error. 
     
     
       6. A method as in  claim 1  wherein the size of the correction charge is fixed. 
     
     
       7. A method comprising:
 (a) estimating an amount of charge entering at a first selected stage in a multiple stage charge transfer pipeline; 
 (b) determining an estimate of an accumulative error introduced at a second selected stage, the second selected stage further along the charge pipeline than the first selected stage; and 
 (c) injecting a correction charge in the pipeline to adjust for the accumulative error, and 
 wherein the size of the correction charge is fixed depending upon an expected charge transfer efficiency. 
 
     
     
       8. A method as in  claim 7  wherein the step of injecting a correction charge comprises digitally adjusting the charge pipeline. 
     
     
       9. A method comprising:
 (a) estimating an amount of charge entering at a first selected stage in a multiple stage charge transfer pipeline; 
 (b) determining an estimate of an accumulative error introduced at a second selected stage, the second selected stage further along the charge pipeline than the first selected stage; and 
 (c) injecting a correction charge in the pipeline to adjust for the accumulative error, and 
 wherein the step of determining the accumulative error additionally comprises: 
 applying a predetermined calibration input charge to the first selected stage; 
 determining an ideal expected charge that should occur at the second selected stage; 
 measuring an actual measured charge at the second selected stage; and 
 determining a difference between the ideal expected charge and the actual measured charge. 
 
     
     
       10. An apparatus comprising:
 (a) a first stage charge detector for determining an amount of charge entering at a first selected stage in the a multiple stage charge pipeline; 
 (b) an accumulative charge error estimator, for estimating an accumulative charge error introduced at a second selected stage in the charge pipeline, the second selected stage further along the charge pipeline than the first selected stage; and 
 (c) a correction charge injector, for injecting a correction charge in the multiple stage charge pipeline to adjust the accumulative charge error for charge transfer inefficiency; 
 wherein the accumulative charge error estimator produces an accumulative charge error that is a function of a per stage transfer efficiency error and the number of stages between the first selected stage and the second selected stage. 
 
     
     
       11. An apparatus as in  claim 10  wherein accumulative charge error estimator further estimates the per stage transfer efficiency as a constant percentage of a corresponding stored charge. 
     
     
       12. An apparatus as in  claim 10  wherein the correction charge injector is coupled to the charge pipeline at a third selected stage which is further along the multiple stage charge pipeline than the second selected stage. 
     
     
       13. An apparatus as in  claim 10  wherein the multiple stage charge transfer device pipeline is a successive approximation charge to digital converter. 
     
     
       14. An apparatus as in  claim 13  wherein the charge to digital converter is a differential charge to digital converter that has a positive “p” side of the pipeline and a minus “m” side of the pipeline, and wherein each side of the multiple stage charge pipeline is corrected for accumulative error. 
     
     
       15. An apparatus as in  claim 10  wherein the size of the correction charge is fixed. 
     
     
       16. An apparatus comprising:
 (a) a first stage charge detector for determining an amount of charge entering at a first selected stage in the a multiple stage charge pipeline; 
 (b) an accumulative charge error estimator, for estimating an accumulative charge error introduced at a second selected stage in the charge pipeline, the second selected stage further along the multiple stage charge pipeline than the first selected stage; and 
 (c) a correction charge injector, for injecting a correction charge in the charge pipeline to adjust the accumulative charge error for charge transfer inefficiency wherein the size of the correction charge is fixed depending upon the expected charge transfer efficiency. 
 
     
     
       17. An apparatus as in  claim 15  wherein the correction charge injector digitally adjusts digital output values from the multiple stage charge pipeline. 
     
     
       18. An apparatus comprising:
 (a) a first stage charge detector for determining an amount of charge entering at a first selected stage in the a multiple stage charge pipeline; 
 (b) an accumulative charge error estimator, for estimating an accumulative charge error introduced at a second selected stage in the multiple stage charge pipeline, the second selected stage further along the charge pipeline than the first selected stage; and 
 (c) a correction charge injector, for injecting a correction charge in the multiple stage charge pipeline to adjust the accumulative charge error for charge transfer inefficiency, 
 wherein the accumulative error estimator further comprises: 
 a calibration input circuit, arranged to couple a predetermined calibration input charge to the first selected stage; 
 a detector, for measuring an actual measured charge at the second selected stage; and 
 a difference detector, for determine a difference between an ideal expected charge and the actual measured charge. 
 
     
     
       19. A method comprising:
 (a) providing an input charge to a first selected stage of a multiple stage charge pipeline;   (b) determining an estimate of an accumulative error in the multiple stage charge pipeline, the estimate being of an accumulative error introduced at a second selected stage in the pipeline, the second selected stage further along the charge pipeline than the first selected stage of the pipeline; and   (c) injecting a correction charge in the pipeline to adjust for the accumulative error, and   wherein the step of determining the accumulative error additionally comprises:
 applying a predetermined calibration input charge to the first selected stage; 
 determining an ideal expected charge that should occur at the second selected stage; 
 measuring an actual measured charge at the second selected stage; and 
 determining a difference between the ideal expected charge and the actual measured charge. 
   
     
     
       20. A method as in claim 19 wherein the correction charge is injected in the multiple stage charge pipeline at a third selected stage which is further along the pipeline than the second selected stage. 
     
     
       21. A method as in claim 19 wherein the multiple stage charge pipeline is a successive approximation charge to digital converter. 
     
     
       22. A method as in claim 21 wherein the charge to digital converter is a differential type charge to digital converter that has a positive “p” side pipeline and a minus “m” side pipeline, and wherein the step of injecting a correction charge injects a correction charge to each side of the pipeline. 
     
     
       23. A method as in claim 19 wherein the size of the correction charge is based on a per stage transfer efficiency. 
     
     
       24. An apparatus comprising:
 (a) a multiple stage charge pipeline having at least a first selected stage and a second selected stage further along the charge pipeline than the first selected stage;   (b) an accumulative charge error estimator, for estimating an accumulative charge error introduced at the second selected stage in the charge pipeline; and   (c) a correction charge injector, for injecting a correction charge in the pipeline to adjust the accumulative charge error for charge transfer inefficiency,   wherein the accumulative error estimator further comprises:   a calibration input circuit, arranged to couple a predetermined calibration input charge to the first selected stage;   a detector, for measuring an actual measured charge at the second selected stage; and   a difference detector, for determine a difference between an ideal expected charge and the actual measured charge.   
     
     
       25. An apparatus as in claim 24 wherein the correction charge injector is coupled to the multiple stage charge pipeline at a third selected stage which is further along the pipeline than the second selected stage. 
     
     
       26. An apparatus as in claim 24 wherein the multiple stage charge pipeline is part of a successive approximation charge to digital converter. 
     
     
       27. An apparatus as in claim 36 wherein the charge to digital converter is a differential charge to digital converter that has a positive “p” side pipeline and a minus “m” side pipeline. 
     
     
       28. A method comprising:
 (a) providing an input charge to a first selected stage of a multiple stage charge pipeline;   (b) determining an estimate of an accumulative error in the multiple stage charge pipeline, the estimate being of an accumulative error introduced at a second selected stage in the pipeline, the second selected stage further along the charge pipeline than the first selected stage of the pipeline; and   (c) injecting a correction charge in the pipeline to adjust for the accumulative error, and   wherein the step of determining an estimate of the accumulative error additionally comprises estimating the accumulative error as a function of a per stage transfer efficiency error and the number of stages between the first selected stage and the second selected stage.   
     
     
       29. A method as in claim 28 wherein the per stage transfer efficiency is estimated as a constant percentage of a corresponding stored charge. 
     
     
       30. A method as in claim 28 wherein the correction charge is injected in the multiple stage charge pipeline at a third selected stage which is further along the pipeline than the second selected stage. 
     
     
       31. A method as in claim 28 wherein the multiple stage charge pipeline is a successive approximation charge to digital converter. 
     
     
       32. A method as in claim 28 wherein the charge to digital converter is a differential charge to digital converter that has a positive “p” side pipeline and a minus “m” side pipeline, and wherein the step of injecting a correction charge injects a corresponding correction charge to each side of the pipeline. 
     
     
       33. An apparatus comprising:
 (a) a multiple stage charge pipeline, connected to receive a charge;   (b) a charge detector for determining an amount of charge entering at a selected stage in the multiple stage charge pipeline;   (c) a charge corrector, for providing a correction charge to the multiple stage charge pipeline at a stage which is further along the pipeline than the selected stage, the correction charge adjusting for a charge transfer inefficiency, and wherein the size of the correction charge is fixed depending upon an expected charge transfer efficiency across multiple stages of the charge pipeline.   
     
     
       34. The apparatus of claim 33 wherein an accumulative charge error estimator further estimates the charge transfer efficiency as a constant percentage of a corresponding stored charge. 
     
     
       35. The apparatus of claim 33 wherein the multiple stage charge pipeline is a successive approximation charge to digital converter. 
     
     
       36. The apparatus of claim 35 wherein the charge to digital converter is a differential charge to digital converter that has a positive “p” pipeline and a minus “m” pipeline, and wherein each side of the pipeline is further coupled to a corresponding charge corrector.