High-speed high-resolution ADC for precision measurements

• US 6,970,118 B2
• Filed: 03/12/2003
• Issued: 11/29/2005
• Est. Priority Date: 08/14/2002
• Status: Active Grant

• Current Assignee: National Instruments Corporation (Emerson Electric Company)
• Original Assignee: National Instruments Corporation (Emerson Electric Company)
• Inventors: Regier, Christopher G.
• Primary Examiner(s): YOUNG, BRIAN K
• Application Number:
  US10/388,296
• Publication Number:
  US 20040034499A1
• Time in Patent Office: 993 Days
• Field of Search: 341/145, 341/144, 341/156, 341/139, 341/131, 341/127, 341/118, 341/120
• US Class Current: 341/118
• 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 1/46 with digital/analogue converter for supplying reference values to converter
• CPC Class Codes: H03M 3/43 the quantiser being a single bit one
• CPC Class Codes: H03M 3/454 with distributed feedback, i.e. with feedback paths from the quantiser output to more than one filter stage
• CPC Class Codes: H03M 3/46 using a combination of at least one delta-sigma modulator in series with at least one analogue/digital converter of a different type

Abstract

A measurement device such as a DMM may include four basic units—an analog circuit path, an analog to digital converter (ADC), a digital filter, and an RMS computation unit. The four basic units may be operable to multiplex or to process one or more of the plurality of channels at the same time. The analog circuit path may include the necessary circuitry for the plurality of channels to couple to one or more analog signals. The analog circuit path may couple to the ADC. The ADC may be operable to receive the one or more analog signals from the analog circuit path and convert it to one or more digital signals. The ADC may include a cascaded ADC, which may include a first ADC and a second ADC. The first and the second ADC and may be able to convert analog data to digital data during a cycle. In one embodiment, the first ADC may generate a conversion result and an error signal. The second ADC may be operable to receive the error signal, digitize the error signal, and process the error signal, thus performing noise cancellation. A summation operation may combine the data from the first ADC and the processed data from the second ADC. The cascaded ADC may consist of a continuous-time 1-bit sigma-delta modulator followed by a SAR ADC. Digital output from sigma-delta modulator may be weighted and summed with the output of the SAR ADC in an FPGA.

Claims

1. A measuring instrument comprising:

a first Analog-to-Digital Converter (ADC) and a second ADC, wherein each of the first and the second ADC comprises an input and an output, wherein the first ADC and the second ADC are operable to convert analog data to digital data during a cycle;

wherein the first ADC is further operable to covert the analog data to the digital data and generate conversion results for the first ADC;

wherein the first ADC is further operable to generate an error signal; and

wherein the second ADC is further operable to sample the error signal, wherein said sampling occurs at a phase of the cycle when a rate of change of the error signal is independent of the conversion results for the first ADC;

wherein the first ADC comprises an internal feedback loop, wherein the internal feedback loop is operable to use return-to-zero coding.

2. The measuring instrument of claim 1,

wherein the measuring instrument further comprises a first logic circuit, wherein the first logic circuit is operable to receive the output of the first ADC and the output of the second ADC, and wherein the first logic circuit is further operable to produce a combined ADC output signal.

3. The measuring instrument of claim 1,

wherein the first ADC further comprises a sigma-delta modulator;

wherein the first ADC further comprises a DAC comprising an input and an output; and

wherein the first ADC further comprises one or more integrators.

4. The measuring instrument of claim 3,

wherein the first ADC further comprises a first integrator and a second integrator, wherein each of the first integrator and the second integrator comprises an input and an output, and wherein the output of the DAC is coupled to the input of the first integrator.

5. The measuring instrument of claim 4,

wherein the output of the first integrator is coupled to the input of the second ADC.

6. The measuring instrument of claim 4,

wherein the first ADC is further operable to output a weighted sum, wherein the weighted sum comprises at least one of the outputs from the first integrator and the second integrator, wherein the second ADC is further operable to receive the weighted sum, wherein the second ADC is further operable to cancel out the error signal using the weighted sum.

7. The measuring instrument of claim 4,

wherein the first ADC further comprises:

a comparator comprising an input and an output, wherein the second integrator is coupled to the input of the comparator;

a second logic circuit comprising one or more of an input, a timing input, and an output, wherein the output of the comparator is coupled to the input of the second logic circuit, wherein the output of the second logic circuit is coupled to the input of the DAC; and

a clock signal coupled to the timing input of the second logic circuit.

8. The measuring instrument of claim 7,

wherein the second logic circuit comprises:

a programmable logic device, comprising one or more of an input, a timing input, and an output, wherein the output of the comparator is coupled to the input of the programmable logic device; and

a flip-flop circuit comprising one or more of an input, a timing input, and an output, wherein the output of the programmable logic device is coupled to the input of the flip-flop circuit.

9. The measuring instrument of claim 8,

wherein the clock signal is generated by an oscillator comprising a clock output, wherein the clock output of the oscillator is coupled to the timing input of the second logic circuit;

and

wherein the clock output of the oscillator is further coupled to the timing input of the programmable logic device and to the timing input of the flip-flop circuit.

10. The measuring instrument of claim 3,

wherein the DAC comprises a first DAC and a second DAC coupled together, wherein each of the first DAC and the second DAC comprises an input and an output.

11. The measuring instrument of claim 10,

wherein the output of the first DAC is coupled to the input of the first integrator, wherein the output of the first DAC is further coupled to the output of the second DAC.

12. The measuring instrument of claim 11,

wherein the first DAC is operable to generate a first output current and the second DAC is operable to generate a second output current, wherein the first output current and the second output current are coupled together to generate a combined output current;

wherein the combined output current is provided to the first integrator;

wherein the combined output current is operable to alter the first output current generated by the first DAC.

13. The measuring instrument of claim 12,

wherein said altering the first output current comprises minimizing the first output current to substantially zero in response to the combined output current.

14. The measuring instrument of claim 10,

wherein the first DAC and the second DAC are coupled together using one or more resistors.

15. The measuring instrument of claim 10,

wherein range of the output of the second DAC is substantially larger than range of the output of the first DAC.

16. The measuring instrument of claim 10,

wherein at least one of the first DAC and the second DAC comprises a one-bit DAC.

17. The measuring instrument of claim 10,

wherein at least one of the first DAC and the second DAC comprises one or more CMOS logic gates.

18. The measuring instrument of claim 1,

wherein the measuring instrument comprises the internal feedback loop;

wherein the internal feedback loop comprises a first-order or higher loop.

19. The measuring instrument of claim 1,

wherein the first ADC comprises a continuous-time one-bit sigma-delta modulator.

20. The measuring instrument of claim 1,

wherein at least one of the first ADC and the second ADC comprises one or more of the following:

a SAR ADC;

an integrating ADC;

a pipeline ADC;

a flash ADC;

a sub-ranging ADC;

a sigma-delta ADC; and

a cascaded ADC.

21. The measuring instrument of claim 1,

wherein the measuring instrument can be one or more of:

a digital multi-meter;

a digital volt-meter; and

any other type of a measuring instrument operable to measure one or more of voltage and current.

22. A high-precision DAC comprising:

a first and a second DAC coupled together, wherein each of the first and the second DAC comprises an input and an output, wherein the output of the second DAC is coupled to the output of the first DAC, wherein the first DAC and the second DAC are each operable to generate and receive a load current, and wherein the load current seen by the first DAC is altered by a changing current load from the second DAC,

wherein the load current received by the first DAC is substantially zero.

23. The high-precision DAC of claim 22,

wherein the first DAC and the second DAC are coupled together using one or more resistors.

24. The high-precision DAC of claim 22,

wherein range of the output of the second DAC is larger than range of the output of the first DAC.

25. The high-precision DAC of claim 22,

wherein the first DAC and the second DAC are comprised in a DMM operable to receive analog data, wherein the first DAC and the second DAC constitute part of an ADC in the DMM.

26. The high-precision DAC of claim 22,

wherein at least one of the first DAC and the second DAC comprises a one-bit DAC.

27. The high-precision DAC of claim 22,

wherein at least one of the first DAC and the second DAC comprises one or more CMOS logic gates.

28. The high-precision DAC of claim 22,

wherein the high-precision DAC is comprised in a measuring instrument, wherein the measuring instrument comprises an internal feedback loop, wherein the measuring instrument further comprises:

a first ADC and a second ADC, wherein the first ADC and the second ADC each comprises an input and an output, wherein the first ADC and the second ADC are operable to convert analog data to digital data during a cycle, wherein the first ADC and the second ADC are coupled together to generate a cascaded ADC;

wherein the first ADC is operable to covert the analog data to the digital data and generate conversion results for the first ADC;

wherein the first ADC is further operable to generate an error signal; and

wherein the second ADC is further operable to sample the error signal, wherein said sampling occurs at a phase of the cycle when a rate of change of the error signal is independent of the conversion results for the first ADC.

29. The high-precision DAC of claim 28,

wherein the measuring instrument further comprises a first logic circuit, wherein the first logic circuit is operable to receive the output of the first ADC and the output of the second ADC, and wherein the first logic circuit is further operable to produce a combined ADC output signal.

30. The high-precision DAC of claim 28,

wherein the first ADC further comprises a sigma-delta modulator;

and

wherein the first ADC further comprises one or more integrators.

31. The high-precision DAC of claim 30,

wherein the first ADC further comprises a first integrator and a second integrator, wherein the output of the first DAC is coupled to the input of the first integrator, and wherein each of the first integrator and the second integrator comprises an input and an output.

32. The high-precision DAC of claim 31,

wherein the output of the first integrator is coupled to the input of the second ADC.

33. The high-precision DAC of claim 31,

wherein the first ADC is further operable to output a weighted sum, wherein the weighted sum comprises at least one of the outputs from the first integrator and the second integrator, wherein the second ADC is further operable to receive the weighted sum, wherein the second ADC is further operable to cancel out the error signal using the weighted sum.

34. The high-precision DAC of claim 31,

wherein the first ADC further comprises:

a comparator comprising an input and an output, wherein the second integrator is coupled to the input of the comparator;

a second logic circuit comprising one or more of an input, a timing input, and an output, wherein the output of the comparator is coupled to the input of the second logic circuit, wherein the output of the second logic circuit is coupled to the one or more of the input of the first DAC and the input of the second DAC; and

a clock signal coupled to the timing input of the second logic circuit.

35. A method for measuring analog data using a measuring instrument, the method comprising:

converting the analog data to digital data during a cycle using a first ADC and a second ADC, wherein each of the first and the second ADC comprises an input and an output, wherein the first ADC is operable to covert the analog data to the digital data and generate conversion results for the first ADC;

the first ADC generating an error signal; and

the second ADC sampling the error signal, wherein said sampling occurs at a phase of the cycle when a rate of change of the error signal is independent of the conversion results for the first ADC;

wherein the first ADC comprises an internal feedback loop, wherein the internal feedback loop is operable to use return-to-zero coding.

36. The method of claim 35,

wherein the measuring instrument comprises a first logic circuit, wherein the first logic circuit is operable to receive the output of the first ADC and the output of the second ADC, and wherein the first logic circuit is further operable to produce a combined ADC output signal.

37. The method of claim 35,

wherein the first ADC further comprises a sigma-delta modulator;

wherein the first ADC further comprises a DAC comprising an input and an output; and

wherein the first ADC further comprises one or more integrators.

38. The method of claim 37,

wherein the first ADC further comprises a first integrator and a second integrator, wherein each of the first integrator and the second integrator comprises an input and an output, and wherein the output of the DAC is coupled to the input of the first integrator.

39. The method of claim 38,

wherein the output of the first integrator is coupled to the input of the second ADC.

40. The method of claim 38,

wherein the first ADC is further operable to output a weighted sum, wherein the weighted sum comprises at least one of the outputs from the first integrator and the second integrator, wherein the second ADC is further operable to receive the weighted sum, wherein the second ADC is further operable to cancel out the error signal using the weighted sum.

41. The method of claim 37,

wherein the DAC comprises a first DAC and a second DAC coupled together, wherein each of the first DAC and the second DAC comprises an input and an output.

42. A measuring instrument comprising:

a first Analog-to-Digital Converter (ADC) and a second ADC, wherein each of the first and the second ADC comprises an input and an output, wherein the first ADC and the second ADC are operable to convert analog data to digital data during a cycle;

wherein the first ADC is further operable to covert the analog data to the digital data and generate conversion results for the first ADC;

wherein the first ADC is further operable to generate an error signal; and

wherein the second ADC is further operable to sample the error signal, wherein said sampling occurs at a phase of the cycle when a rate of change of the error signal is independent of the conversion results for the first ADC;

wherein the first ADC further comprises a sigma-delta modulator;

wherein the first ADC further comprises a DAC comprising an input and an output;

wherein the first ADC further comprises one or more integrators; and

wherein the DAC comprises a first DAC and a second DAC coupled together, wherein each of the first DAC and the second DAC comprises an input and an output.

43. The measuring instrument of claim 42,

wherein the measuring instrument further comprises a first logic circuit, wherein the first logic circuit is operable to receive the output of the first ADC and the output of the second ADC, and wherein the first logic circuit is further operable to produce a combined ADC output signal.

44. The measuring instrument of claim 42,

wherein the first ADC further comprises a sigma-delta modulator;

wherein the first ADC further comprises a DAC comprising an input and an output; and

wherein the first ADC further comprises one or more integrators.

45. The measuring instrument of claim 42,

wherein the measuring instrument comprises an internal feedback loop;

wherein the first ADC comprises the internal feedback loop, wherein the internal feedback loop comprises a first-order or higher loop.

46. The measuring instrument of claim 45,

wherein the internal feedback loop is operable to use return-to-zero coding.

47. The measuring instrument of claim 42,

wherein the first ADC comprises a continuous-time one-bit sigma-delta modulator.

48. The measuring instrument of claim 42,

wherein the first ADC further comprises a first integrator and a second integrator, wherein each of the first integrator and the second integrator comprises an input and an output, and wherein the output of the DAC is coupled to the input of the first integrator.

49. A measuring instrument comprising:

a first Analog-to-Digital Converter (ADC) and a second ADC, wherein each of the first and the second ADC comprises an input and an output, wherein the first ADC and the second ADC are operable to convert analog data to digital data during a cycle;

wherein the first ADC is further operable to convert the analog data to the digital data and generate conversion results for the first ADC;

wherein the first ADC is further operable to generate an error signal; and

wherein the second ADC is further operable to sample the error signal, wherein said sampling occurs at a phase of the cycle when a rate of change of the error signal is independent of the conversion results for the first ADC;

wherein the first ADC further comprises a sigma-delta modulator;

wherein the first ADC further comprises a DAC comprising an input and an output;

wherein the first ADC further comprises one or more integrators; and

wherein the first ADC further comprises a first integrator and a second integrator, wherein each of the first integrator and the second integrator comprises an input and an output, and wherein the output of the DAC is coupled to the input of the first integrator.