US 20050275572 A1 Abstract An analog-to-digital (ADC) converter circuit that converts an analog input signal into a digital output circuit includes a calibration coefficient computation circuit for computing calibration coefficients of a calibration filter. The calibration coefficient computation circuit includes a switching device adapted to switch the analog input signal delivered to the ADC circuit between on and off states, and includes a pseudo-random signal generator adapted to input a pseudo-random signal to the ADC circuit. During a start-up phase of the ADC circuit, the ADC circuit, the switching device turns off the analog input signal to the ADC circuit, the pseudo-random signal generator inputs a pseudo-random signal into the ADC circuit, and the calibration coefficient computation circuit computes the calibration coefficients of the calibration filter. This ADC circuit configuration reduces startup time for the calibration filter to only a few clock cycles.
Claims(37) 1. A method of calibrating an analog-to-digital converter circuit, comprising:
disabling an input signal to the analog-to-digital converter circuit; inputting a pseudo-random signal to the analog-to-digital converter circuit; measuring at least one of estimated gain and estimated integrator pole of the analog-to-digital converter circuit; and calculating coefficients of a calibration filter used in the analog-to-digital converter circuit using at least one of the estimated gain and the estimated integrator pole. 2. A method of enabling the input signal of the analog-to-digital converter after calculating the coefficients of the calibration filter used in the analog-to-digital converter circuit; and operating the analog-to-digital converter circuit in a continuous calibration mode. 3. A method of 4. A method of multiplying a digitized residue of a first stage of the analog-to-digital converter circuit with the pseudo-random signal; accumulating a sum of the multiplication; and scaling the sum by an expected magnitude of the pseudo-random signal. 5. A method of 6. A method of 7. A method of 8. A method of 9. A method of measuring the estimated gain of the analog-to-digital converter circuit by:
resetting an integrator of the analog-to-digital converter circuit for every clock period;
multiplying a digitized residue of a first stage of the analog-to-digital converter circuit with the pseudo-random signal to get a first multiplication,
accumulating the first multiplication to get a first sum, and
scaling the first sum by an expected magnitude of the pseudo-random signal; and
measuring the integrator pole of the analog-to-digital converter circuit by:
resetting the integrator of the analog-to-digital converter circuit for every second clock period;
multiplying a digitized residue of a first stage of the analog-to-digital converter circuit with a memory of the pseudo-random signal to get a second multiplication,
accumulating the second multiplication to get a second sum,
accumulating a third sum of ratios of a sign of the pseudo-random signal and the sign of the memory of the pseudo-random signal,
subtracting the third sum from the second sum to get a first difference, and
scaling the first difference by a number of measurements.
10. A method of 11. A method of 12. A method of measuring calibration coefficients of a calibration filter used in a pipelined analog-to-digital converter circuit, the pipelined analog-to-digital converter circuit having a first analog-to-digital converter, a second analog-to-digital converter and a digital-to analog converter in a feed-back path of the first analog-to-digital converter, the method comprising:
disabling an input signal to the pipelined analog-to-digital converter circuit; inputting a pseudo-random signal to the pipelined analog-to-digital converter circuit; inputting a gained output of the digital-to-analog converter to the second analog-to-digital converter; and inputting the pseudo-random signal and the output of the second analog-to-digital converter to a calibration coefficient computation circuit to thereby calculate calibration coefficients. 13. A method of 14. A method of 15. A method of 16. A method of measuring calibration coefficients of a calibration filter used in a ΔΣ analog-to-digital converter circuit, the ΔΣ analog-to-digital converter circuit having a first analog-to-digital converter, a second analog-to-digital converter and a digital-to analog converter in a feed-back path of the first analog-to-digital converter, the method comprising:
disabling an input signal to the ΔΣ analog-to-digital converter circuit; inputting a pseudo-random signal to the ΔΣ analog-to-digital converter circuit; inputting an integrated output of the digital-to-analog converter to the second analog-to-digital converter; and inputting the pseudo-random signal and the output of the second analog-to-digital converter to a calibration coefficient computation circuit to thereby calculate calibration coefficients. 17. A method of 18. A method of 19. A method of 20. A method of resetting an integrator of the analog-to-digital converter circuit for every clock period for calculating the estimated gain of the ΔΣ analog-to-digital converter circuit; and resetting an integrator of the analog-to-digital converter circuit for every second clock period for calculating the estimated gain of the ΔΣ analog-to-digital converter circuit. 21. An analog-to-digital converter system for converting an analog input signal to a digital output signal, the circuit comprising:
an analog-to-digital converter circuit; a switching device for turning on and turning off the analog input signal to the analog-to-digital converter circuit; a pseudo-random signal generator providing a pseudo-random signal to the analog-to-digital converter circuit, a calibration coefficient computation circuit to compute calibration coefficients of the analog-to-digital converter circuit; and a calibration filter circuit for filtering the digital output signal according to the calibration coefficients. 22. An analog-to-digital converter system of 23. An analog-to-digital converter system of 24. An analog-to-digital converter system of 25. An analog-to-digital converter system of 26. An analog-to-digital converter system of 27. An analog-to-digital converter system of 28. An analog-to-digital converter system of 29. An analog-to-digital converter system of 30. An analog-to-digital converter system of 31. A calibration coefficient computation circuit for computing calibration coefficients of a calibration filter used in an analog-to-digital converter circuit, the analog-to-digital circuit converting an analog input signal into a digital output signal, the calibration coefficient computation circuit comprising:
a switching device adapted to turn on and turn off the analog input signal to the analog-to-digital converter and a pseudo-random signal generator adapted to input a pseudo-random signal to the analog-to-digital converter circuit, wherein the switching device turns of the analog input signal to the analog-to-digital converter circuit during a startup phase of the analog-to-digital converter circuit, the pseudo-random signal generator inputs a pseudo-random input signal into the analog-to-digital converter circuit during the startup phase and the calibration coefficient computation circuit computes the calibration coefficients of the calibration filter during the startup phase. 32. A calibration coefficient computation circuit of 33. A calibration coefficient computation circuit of 34. A calibration coefficient computation circuit of 35. A calibration coefficient computation circuit of 36. A calibration coefficient computation circuit of 37. A calibration coefficient computation circuit of Description This patent relates generally to analog-to-digital converters, and more specifically to an apparatus and a method for calibrating analog-to-digital converters. Analog-to-digital converters (ADCs) are employed in a variety of electronic systems including computer modems, wireless telephones, satellite receivers, process control systems, etc. Such systems demand cost-effective ADCs that can efficiently convert an analog input signal to a digital output signal over a wide range of frequencies and signal magnitudes with minimal noise and distortion. An ADC typically converts an analog signal to a digital signal by sampling the analog signal at pre-determined sampling intervals and generating a sequence of binary numbers via a quantizer, wherein the sequence of binary numbers is a digital representation of the sampled analog signal. Some of the commonly used types of ADCs include integrating ADCs, Flash ADCs, pipelined ADCs, successive approximation register ADCs, Delta-Sigma (ΔΣ) ADCs, two-step ADCs, etc. Of these various types, the pipelined ADCs and the ΔΣ ADCs are particularly popular in applications requiring higher resolutions. A pipelined ADC circuit samples an analog input signal using a sample-and-hold circuit to hold the input signal steady and a first stage flash ADC to quantize the input signal. The first stage flash ADC then feeds the quantized signal to a digital-to-analog converter (DAC). The pipelined ADC circuit subtracts the output of the DAC from the analog input signal to get a residue signal of the first stage. The first stage of the pipelined ADC circuit generates the most significant bit (MSB) of the digital output signal. The residue signal of the first stage is gained up by a factor and fed to the next stage. Subsequently, the next stage of the pipelined ADC circuit further quantizes the residue signal to generate a further bit of the digital output signal, with this process being repeated for each stage of the ADC circuit. On the other hand, a ΔΣ ADC employs over-sampling, noise-shaping, digital filtering and digital decimation techniques to provide high resolution analog-to-digital conversion. One popular design of a ΔΣ ADC is a multi-stage noise shaping (MASH) ΔΣ ADC. A MASH ΔΣ ADC is based on cascading multiple first-order or second-order ΔΣ ADCs to realize high-order noise shaping. While both pipelined ADCs and ΔΣ ADCs provide improved signal-to-noise ratio, improved stability, etc., the performance of both pipelined ADCs and ΔΣ ADCs is bottlenecked by the linearity of the internal DAC. For example, the gain error of a DAC used in the first stage of a pipelined ADC circuit contributes to the overall gain error of the pipelined ADC circuit. The gain error of an ADC can be defined as the amount of deviation between an ideal transfer function and a measured transfer function of the ADC. One method used to overcome the limitations imposed by the gain errors of various stages of ADCs is to digitally calibrate the gain errors using calibration filters. An illustration of a known pipelined ADC circuit The pipelined ADC circuit The pseudo-random signal d While the calibration filter One of the disadvantages with the implementations of calibration filters using an iterative algorithm described above is the long time, usually on order of million clock cycles, that is necessary for the calibration filters to converge to a correct set of filter coefficients. Specifically, the calibration filter Such a long startup time for computation of calibration coefficients results in a requirement for longer testing time for circuits using ADC components, sometimes over a minute for each component, which is a major problem for volume production of circuits using ADC components. To facilitate volume production of circuits using ADC components at a reasonable cost, it is necessary to reduce the startup time required for ADC calibration filters. The present patent is illustrated by way of examples and not limitations in the accompanying figures, in which like references indicate similar elements, and in which: An analog-to-digital (ADC) converter circuit that converts an analog input signal into a digital output signal includes a calibration coefficient computation circuit for computing calibration filter coefficients. The calibration coefficient computation circuit includes a switching device adapted to switch the analog input signal delivered to the ADC circuit between on and off states, and includes a pseudo-random signal generator adapted to input a pseudo-random signal to the ADC circuit. During a start-up phase of the ADC circuit, the switching device turns off the analog input signal to the ADC circuit, the pseudo-random signal generator inputs a pseudo-random signal into the ADC circuit, and the calibration coefficient computation circuit computes the calibration coefficients of the calibration filter. This ADC circuit configuration reduces startup time for the calibration filter to only a few clock cycles. While the calibration coefficient computation circuit is described herein with a pipelined ADC circuit and a MASH ΔΣ ADC circuit, it may also be used with various other types of ADC circuits. Now referring to At the start of the fast startup method illustrated by the flowchart Upon calculating the estimated gain and/or the estimated integrator pole of the ADC A block The first stage The pipelined ADC circuit The pipelined ADC circuit On the other hand, during the startup phase, when the switching device The difference of the analog input signal u and an output d Given the ideal gain of the pipelined ADC circuit Subsequently, the output signal d If the filter coefficient of the gain filter For the pipelined ADC circuit Rearranging the equation above:
Thus, from the results above, for the pipelined ADC circuit An estimate of gain G may be found by applying the fast startup method illustrated in the flowchart For illustration purpose, suppose that the ADC An implementation of the calibration coefficient computation circuit To calculate the value of estimated gain G, a multiplier
Where the gain is calculated as follows:
Subsequently, the output of the GEC circuit While the circuits of The first stage The MASH ΔΣ ADC circuit On the other hand, during the startup phase, when the switching device For the MASH ΔΣ ADC circuit This leads to simultaneous requirements that the sum of all factors not containing z Thus, in order to calculate the filter coefficients l An estimate of the gain G may be found by applying the fast startup method of the flowchart Similarly, an estimate of the integrator pole p can be found by shorting the analog input signal u to zero (using the switching device An estimate of p may be found by rewriting the above equation and using the previous estimate of gain G, as follows:
Because the digital signal d Subsequently, the estimated value of p can be obtained by dividing the sum of p by a number of observations used in obtaining the sum, as follows:
The following table provides an example of a computation of an estimated gain G and estimated integrator pole p for the MASH ΔΣ ADC circuit
Where the estimated integrator pole p is calculated as follows:
An implementation of the calibration coefficient computation circuit The operation of the l Subsequently, a first correlator A summation circuit Although the forgoing text sets forth a detailed description of numerous different embodiments of the invention, it should be understood that the scope of the invention is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possible embodiment of the invention because describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims defining the invention. Thus, many modifications and variations may be made in the techniques and structures described and illustrated herein without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the methods and apparatus described herein are illustrative only and are not limiting upon the scope of the invention. Referenced by
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