SAR ADC providing digital codes with high accuracy and high throughput performance

• US 6,958,722 B1
• Filed: 06/11/2004
• Issued: 10/25/2005
• Est. Priority Date: 06/11/2004
• Status: Active Grant

• Current Assignee: Conmed Corporation (Jefferies Financial Group, Inc.)
• Original Assignee: Texas Instruments, Inc.
• Inventors: Varma, Vikram, Mitikiri, Yujendra, Janakiraman, Seetharaman
• Primary Examiner(s): YOUNG, BRIAN K
• Application Number:
  US10/709,996
• Time in Patent Office: 501 Days
• Field of Search: 341/156, 341/155, 341/158, 341/161, 341/162, 341/163, 341/141, 341/145, 341/172, 341/146, 307/240, 307/241, 307/243, 307/246
• US Class Current: 341/161
• CPC Class Codes: H03M 1/144 the steps being performed sequentially in a single stage, i.e. recirculation type H03M1/141, H03M1/143, H03M1/16 take precedence
• CPC Class Codes: H03M 1/145 the steps being performed sequentially in series-connected stages H03M1/141, H03M1/143, H03M1/16 take precedence
• CPC Class Codes: H03M 1/468 in which the input S/H circuit is merged with the feedback DAC array

Abstract

An aspect of the invention improves accuracy of digital codes generated at the output of a SAR ADC by using multiple reference voltages. A first reference voltage is used to generate an equivalent voltage corresponding to previous resolved bits and a second reference voltage is used to generate equivalent voltage corresponding to the bits being presently resolved. Another aspect of the present invention provides an ADC with high SNR as well as high throughput performance. Such a feature may be achieved by resolving some of the MSBs of the digital code using a high speed and low SNR DAC and remaining bits of the digital code using a high SNR DAC.

Claims

1. A method of converting a sample of an analog signal to a N-bit digital code, said method being performed in an analog to digital converter (ADC), said method comprising:

receiving said sample of said analog signal;

resolving P most significant bits (MSBs) of said N-bit digital code from said sample using one of a first reference voltage and a second reference voltage, wherein P is less than N; and

resolving Q bits of said N-bit digital code while using said first reference voltage to generate an equivalent voltage corresponding to said P MSBs and using said second reference voltage to generate an equivalent voltage corresponding to said Q bits,

wherein said Q bits form the next MSBs following said P MSBs,

further comprising resolving next R bits of said N-bit digital code while using said first reference voltage to generate an equivalent voltage corresponding to both of said P MSBs and said Q bits, and said second reference voltage to generate an equivalent voltage corresponding to said R bits.

2. A method of converting a sample of an analog signal to a N-bit digital code, said method being performed in an analog to digital converter (ADC), said method comprising:

receiving said sample of said analog signal;

resolving P most significant bits (MSBs) of said N-bit digital code from said sample using one of a first reference voltage and a second reference voltage, wherein P is less than N; and

resolving Q bits of said N-bit digital code while using said first reference voltage to generate an equivalent voltage corresponding to said P MSBs and using said second reference voltage to generate an equivalent voltage corresponding to said Q bits,

wherein said Q bits form the next MSBs following said P MSBs,

further comprises generating said first reference voltage using a first buffer and said second reference voltage using a second buffer.

3. The method of claim 2, wherein the voltage level of said first reference voltage equals the voltage level of said second reference voltage.

4. The method of claim 3, wherein said ADC comprises N capacitors operated according to successive approximation principle (SAP), wherein said N capacitors comprise P capacitors to resolve said P MSBs and Q capacitors to resolve said Q bits, wherein said resolving P MSBs comprises:

sampling said sample on said N capacitors in a sampling phase; and

connecting each of said P capacitors to said first reference voltage or ground based on a corresponding bit of said N_—

bit digital code in each iteration.

5. The method of claim 4, wherein said resolving Q bits comprises connecting each of said Q capacitors to said second reference voltage or ground based on a corresponding bit of said N_—

bit digital code in each iteration, wherein each of said Q capacitors are connected to said first reference voltage or ground after resolving said Q bits based on a corresponding bit of said N_—

bit digital code in each iteration.

6. The method of claim 5, further comprises correcting an error in said P MSBs, said correcting comprises:

connecting a first capacitor to a ground before completing resolving of said P MSBs;

connecting a second capacitor to said first reference voltage before completing resolving of said P MSBs, wherein said first capacitor and said second capacitor are provided for said correcting; and

connecting said first capacitor to said first reference voltage if a voltage representing said P MSBs is less than a voltage level of said sample after resolving of said P MSBs, else connecting said second capacitor to ground.

7. The method of claim 6, further comprises correcting an error in said Q bits, wherein said correcting said error in said Q bits comprises:

connecting a third capacitor to a ground before completing resolving of said Q bits;

connecting a fourth capacitor to said second reference voltage before completing resolving of said Q bits, wherein said third capacitor and said fourth capacitor are provided for said correcting said error in said Q bits; and

connecting said third capacitor to said first reference voltage if a voltage representing said P MSBs and said Q bits is less than said voltage level of said sample after resolving of said Q bits, else connecting said fourth capacitor to ground.

8. A method of converting a sample of an analog signal to a N-bit digital code, said method being performed in an analog to digital converter (ADC), said method comprising:

receiving said sample;

resolving at least one most significant bits (MSB) of said N-bit digital code using a first DAC;

setting an intermediate digital value to equal said at least one MSB in corresponding bit positions; and

resolving at least one of the remaining bits of said MSB using a second DAC starting from said intermediate digital value,

wherein each of said at least one MSB and at least one of the remaining bits are resolved according to a successive approximation principle (SAP).

9. The method of claim 8, wherein said at least one MSB comprise M most significant bits (MSBs) of said N-bit digital code, wherein said at least one of the remaining bits comprise remaining (N−

M bits) of said N-bit digital code.

10. The method of claim 9, wherein said first DAC is implemented with a higher speed compared to said second DAC, and said second DAC is implemented with a higher SNR compared to said first DAC such that said ADC is provided with high speed and high SNR.

11. The method of claim 10, wherein said first DAC comprises a first plurality of capacitors and said second DAC comprises a second plurality of capacitors, wherein a capacitance value of each of said second plurality of capacitors is greater than the capacitance value of a corresponding one of said first plurality of capacitors.

12. The method of claim 11, further comprises:

providing a first reference voltage and a second reference voltage to said second DAC, wherein said first reference voltage and said second reference voltage are generated by a first buffer and a second buffer respectively.

13. A successive approximation type analog to digital converter (SAR ADC) converting a sample of an input analog signal into an N-bit digital code, said SAR ADC comprising:

a SAR logic determining said N-bit digital code, said SAR logic providing an intermediate digital value in each iteration according to a comparison result which is based on a comparison of a voltage level equivalent of the intermediate digital value provided in a prior iteration with said sample;

a comparator providing said comparison result; and

a digital to analog converter (DAC) converting said intermediate value to an equivalent voltage level, said DAC receiving a first reference voltage and a second reference voltage, wherein said DAC generates said voltage level using said first reference voltage for a first subset of bits and said second reference voltage for a second subset of bits, wherein said first subset of bits and said second subset of bits are contained in said intermediate digital value.

14. The SAR ADC of claim 13, wherein said voltage level represents a sum of a first voltage portion and a second voltage portion, wherein said first voltage portion represents an equivalent voltage corresponding to said first subset of bits and said second voltage portion represents an equivalent voltage corresponding to said second subset of bits.

15. The SAR ADC of claim 14, wherein said first subset of bits comprise P most significant bits (MSBs) resolved in prior iterations, and said second subset of bits comprise Q bits following said P MSBs, wherein said Q bits are presently being resolved by said SAR ADC.

16. The SAR ADC of claim 15, wherein an equivalent voltage corresponding to both of said P MSBs and said Q bits is generated using said first reference voltage in a subsequent iteration.

17. The SAR ADC of claim 16, further comprising a first reference buffer providing said first reference voltage and a second reference buffer providing said second reference voltage.

18. The SAR ADC of claim 16, wherein said DAC receives said sample and generates (a second input voltage−

voltage of said sample+equivalent voltage of said intermediate digital value) as an input to said comparator, wherein ‘

−

’

represents a subtraction operation and said second input voltage is also provided as an input to said comparator.

19. The SAR ADC of claim 16, wherein said DAC generates an equivalent voltage corresponding to a next R bits of said N-bit digital code using said second reference voltage.

20. The SAR ADC of claim 19, wherein the voltage level of said first reference voltage equals the voltage level of said second reference voltage.

21. The SAR ADC of claim 19, wherein said DAC comprises N capacitors, wherein said N capacitors comprise P capacitors to resolve said P MSBs and Q capacitors to resolve said Q bits, wherein said N capacitors are connected to said sample in a sampling phase, and wherein each of said Q capacitors is connected to said second reference voltage or ground based on a corresponding bit of said N_—

bit digital code in said present iteration and to said first reference voltage or ground based on a corresponding bit of said N-bit digital code in said subsequent iteration.

22. The SAR ADC of claim 21, further comprises a correction circuit correcting an error in said P MSBs, said correction circuit comprises:

a first capacitor connected to a ground before completing resolving of said P MSBs; and

a second capacitor connected to said first reference voltage before completing resolving of said P MSBs,

wherein said first capacitor is connected to said first reference voltage if a voltage representing said P MSBs is less than a voltage level of said sample after resolving of said P MSBs, else said second capacitor is connected to ground.

23. A successive approximation type analog to digital converter (SAR ADC) converting a sample of an input analog signal into an N-bit digital code, said SAR ADC comprising:

a first digital to analog converter (DAC) converting a first input to an equivalent voltage level;

a second digital to analog converter (DAC) converting a second input to an equivalent voltage level wherein said second input comprises N-bits; and

a SAR logic sending a first sequence of input bit sets as said first input and determining a corresponding number of most significant bit (MSB) values, wherein each of said first sequence of input bit sets contains a number of bits equaling a number of bits in said first input, said SAR logic setting corresponding MSB positions of an intermediate digital value to the determined MSB values and sending said intermediate digital value as said second input to determine another bit of said N-bit digital code.

24. The SAR ADC of claim 23, further comprising

a first comparator providing a first comparison result which is based on a comparison of said equivalent voltage of said first input with said sample;

a second comparator providing a second comparison result which is based on a comparison of said equivalent voltage of said second input with said sample; and

a multiplexor sending providing said first comparison result as an output when said SAR logic sends said first sequence of input bit sets as said first input, and said second comparison result as said output when said SAR logic sends said intermediate digital value as said second input, wherein said SAR logic determines each bit of said N-bit digital code according to said output.

25. The SAR ADC of claim 23, wherein said first DAC is implemented with a higher speed compared to said second DAC, and said second DAC is implemented with a higher SNR compared to said first DAC such that said ADC is provided with high speed and high SNR.

26. The SAR ADC of claim 25, wherein said first DAC comprises a first plurality of capacitors and said second DAC comprises a second plurality of capacitors, wherein a capacitance value of each of said second plurality of capacitors is greater than the capacitance value of a corresponding one of said first plurality of capacitors.

27. The SAR ADC of claim 26, further comprises:

a first buffer and a second buffer, wherein said first buffer and said second buffer respectively provides a first reference voltage and a second reference voltage to said second DAC.

28. A system comprising:

an analog processor processing an analog signal to generate a sample of an analog signal;

a successive approximation digital to analog converter (SAR ADC) converting said sample into an N-bit digital code, said SAR ADC comprising;

a SAR logic determining said N-bit digital code, said SAR logic providing an intermediate digital value in each iteration according to a comparison result which is based on a comparison of a voltage level equivalent of the intermediate digital value provided in a prior iteration with said sample;

a comparator providing said comparison result; and

a digital to analog converter (DAC) converting said intermediate value to an equivalent voltage level, said DAC receiving a first reference voltage and a second reference voltage, wherein said DAC generates said voltage level using said first reference voltage for a first subset of bits and said second reference voltage for a second subset of bits, wherein said first subset of bits and said second subset of bits are contained in said intermediate digital value.

29. The system of claim 28, wherein said voltage level represents a sum of a first voltage portion and a second voltage portion, wherein said first voltage portion represents an equivalent voltage corresponding to said first subset of bits and said second voltage portion represents an equivalent voltage corresponding to said second subset of bits.

30. The system of claim 29, wherein said first subset of bits comprise P most significant bits (MSBs) resolved in prior iterations, and said second subset of bits comprise Q bits following said P MSBs, wherein said Q bits are presently being resolved by said SAR ADC.

31. The system of claim 30, wherein an equivalent voltage corresponding to both of said P MSBs and said Q bits is generated using said first reference voltage in a subsequent iteration.

32. The system of claim 30, further comprising a first reference buffer providing said first reference voltage and a second reference buffer providing said second reference voltage.

33. The system of claim 30, wherein said DAC receives said sample and generates (a second input voltage−

voltage of said sample+equivalent voltage of said intermediate digital value) as an input to said comparator, wherein ‘

−

’

represents a subtraction operation and said second input voltage is also provided as an input to said comparator.

34. The system of claim 30, wherein said DAC generates an equivalent voltage corresponding to a next R bits of said N-bit digital code using said second reference voltage.

35. The system of claim 34, wherein the voltage level of said first reference voltage equals the voltage level of said second reference voltage.

36. The system of claim 35, wherein said DAC comprises N capacitors, wherein said N capacitors comprise P capacitors to resolve said P MSBs and Q capacitors to resolve said Q bits, wherein said N capacitors are connected to said sample in a sampling phase, and wherein each of said Q capacitors is connected to said second reference voltage or ground based on a corresponding bit of said N_—

bit digital code in said present iteration and to said first reference voltage or ground based on a corresponding bit of said N-bit digital code in said subsequent iteration.

37. The system of claim 36, further comprises a correction circuit correcting an error in said P MSBs, said correction circuit comprises:

a first capacitor connected to a ground before completing resolving of said P MSBs; and

a second capacitor connected to said first reference voltage before completing resolving of said P MSBs,

wherein said first capacitor is connected to said first reference voltage if a voltage representing said P MSBs is less than a voltage level of said sample after resolving of said P MSBs, else said second capacitor is connected to ground.

38. The system of claim 37, wherein said system comprises a global positioning system receiver, said system further comprising an antenna to receive said analog signal and provide said analog signal to said analog processor.

39. A system comprising:

an analog processor processing an analog signal to generate a sample of an analog signal;

a successive approximation type analog to digital converter (SAR ADC) converting said sample into an N-bit digital code, said SAR ADC comprising;

a first comparator providing a comparison result of a first analog signal and said sample;

a second comparator providing a comparison result of a second analog signal and said sample;

a first digital to analog converter (DAC) receiving said sample and an intermediate P-bit digital value, said first DAC generating said first analog signal based on said intermediate P-bit digital value in each iteration;

a second DAC receiving said sample and an intermediate N-bit digital value, said second DAC generating said second analog signal based on said intermediate N-bit digital value in each iteration; and

a SAR logic determining said N-bit digital code by resolving a first subset of bits by interfacing with said first DAC and said first comparator, and then resolving a second subset of bits by interfacing with said second DAC and said second comparator, wherein said first subset of bits and said second subset of bits are used to generate said N-bit digital code.

40. The system of claim 39, wherein said first subset of bits comprise M most significant bits (MSBs) of said N-bit digital code, wherein said second subset comprises remaining (N−

M bits) of said N-bit digital code.

41. The system of claim 40, wherein said first DAC is implemented with a higher speed compared to said second DAC, and said second DAC is implemented with a higher SNR compared to said first DAC such that said ADC is provided with high speed and high SNR.

42. The system of claim 41, wherein said first DAC comprises a first plurality of capacitors and said second DAC comprises a second plurality of capacitors, wherein a capacitance value of each of said second plurality of capacitors is greater than the capacitance value of a corresponding one of said first plurality of capacitors.

43. The system of claim 42, further comprises:

a first buffer and a second buffer, wherein said first buffer and said second buffer respectively provides a first reference voltage and a second reference voltage to said second DAC.

44. An apparatus converting a sample of an analog signal to a N-bit digital code, said apparatus comprising:

means for receiving said sample of said analog signal;

means for resolving P most significant bits (MSBs) of said N-bit digital code from said sample using one of a first reference voltage and a second reference voltage, wherein P is less than N; and

means for resolving Q bits of said N-bit digital code while using said first reference voltage to generate an equivalent voltage corresponding to said P MSBs and using said second reference voltage to generate an equivalent voltage corresponding to said Q bits,

wherein said Q bits form the next MSBs following said P MSBs,

further comprises means for resolving next R bits of said N-bit digital code while using said first reference voltage to generate an equivalent voltage corresponding to both of said P MSBs and said Q bits, and said second reference voltage to generate an equivalent voltage corresponding to said R bits.

45. The apparatus of claim 44, further comprises means for generating said first reference voltage using a first buffer and said second reference voltage using a second buffer.

46. The apparatus of claim 45, wherein the voltage level of said first reference voltage equals the voltage level of said second reference voltage.

47. The apparatus of claim 46, further comprises an analog to digital converter (ADC), wherein said ADC comprises N capacitors operated according to successive approximation principle (SAP), wherein said N capacitors comprise P capacitors to resolve said P MSBs and Q capacitors to resolve said Q bits, wherein said means for resolving P MSBs comprises:

means for sampling said sample on said N capacitors in a sampling phase; and

means for connecting each of said P capacitors to said first reference voltage or ground based on a corresponding bit of said N_—

bit digital code in each iteration.

48. The apparatus of claim 47, wherein said resolving Q bits comprises means for connecting each of said Q capacitors to said second reference voltage or ground based on a corresponding bit of said N_—

bit digital code in each iteration, wherein each of said Q capacitors are connected to said first reference voltage or ground after resolving said Q bits based on a corresponding bit of said N_—

bit digital code in each iteration.

49. The apparatus of claim 48, further comprises means for correcting an error in said P MSBs, said means for correcting comprises:

means for connecting a first capacitor to a ground before completing resolving of said P MSBs;

means for connecting a second capacitor to said first reference voltage before completing resolving of said P MSBs, wherein said first capacitor and said second capacitor are provided for said means for correcting; and

means for connecting said first capacitor to said first reference voltage if a voltage representing said P MSBs is less than a voltage level of said sample after resolving of said P MSBs, else means for connecting said second capacitor to ground.

50. An apparatus converting a sample of an analog signal to a N-bit digital code, said apparatus comprising:

means for receiving said sample of a voltage level;

means for providing said sample of said voltage level as an input to a first digital to analog converter (DAC) to resolve a first subset of bits; and

means for providing said sample of said voltage level as an input to a second DAC to resolve a second subset of bits, wherein said first subset of bits and said second subset of bits are used to generate said N-bit digital code,

wherein said first subset of bits comprise M most significant bits (MSBs) of said N-bit digital code, wherein said second subset comprises remaining (N−

M bits) of said N-bit digital code, and

wherein said first DAC is implemented with a higher speed compared to said second DAC, and said second DAC is implemented with a higher SNR compared to said first DAC such that said ADC is provided with high speed and high SNR.

51. The apparatus of claim 50, wherein said first DAC comprises a first plurality of capacitors and said second DAC comprises a second plurality of capacitors, wherein a capacitance value of each of said second plurality of capacitors is greater than the capacitance value of a corresponding one of said first plurality of capacitors.

52. The apparatus of claim 51, further comprises:

means for providing a first reference voltage and a second reference voltage to said second DAC, wherein said first reference voltage and said second reference voltage are generated by a first buffer and a second buffer respectively.