Digital-to-analog converter with digital linearity correction
First Claim
1. A method of operating a digital-to-analog converter, said method comprising the steps of:
- a) generating a digital calibration signal having at least one frequency component;
b) operating said digital-to-analog converter in a calibration mode to convert said digital calibration signal to produce an analog signal;
c) detecting, in said analog signal, a frequency component generated from said digital calibration signal by nonlinearity, to produce a compensation coefficient; and
d) operating said digital-to-analog converter in a conversion mode in which a digital input signal is digitally compensated by said compensation coefficient to produce a distorted digital signal, said compensation coefficient specifying a degree of nonlinear distortion of said digital input signal produced in said distorted digital signal, and in which the distorted digital signal is converted by said digital-to-analog converter to produce an analog signal having reduced nonlinear distortion.
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Abstract
Digital linearity correction is obtained by generating a digital calibration signal having at least one frequency component, converting the digital calibration signal to an analog signal, and detecting distortion in the analog signal generated from the calibration signal by nonlinearity to produce a compensation coefficient used to digitally compensate the digital input of the digital-to-analog converter. The compensation coefficient is adjusted in a feedback loop so that the distortion is minimized. Preferably the calibration signal has two frequencies, and the distortion is an intermodulation component having a substantially lower frequency. The intermodulation component, for example, is selected by an R-C low-pass filter, digitized by an analog-to-digital converter, and detected by digital signal processing. The analog-to-digital converter may have low resolution, low dynamic range and a low sampling rate. Preferably the digital compensation includes computing a third-order polynomial, and the intermodulation components for the second and third order terms are detected at about the same frequency. Preferably the offset of the digital-to-analog converter is calibrated first by setting the digital input to zero, modulating the analog output at about the same frequency, and adjusting the offset to eliminate any signal from the modulator. The calibration method can also be used to compensate for nonlinearity in signal conditioning circuitry, or to obtain a specified non-linear transfer characteristic.
104 Citations
59 Claims
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1. A method of operating a digital-to-analog converter, said method comprising the steps of:
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a) generating a digital calibration signal having at least one frequency component; b) operating said digital-to-analog converter in a calibration mode to convert said digital calibration signal to produce an analog signal; c) detecting, in said analog signal, a frequency component generated from said digital calibration signal by nonlinearity, to produce a compensation coefficient; and d) operating said digital-to-analog converter in a conversion mode in which a digital input signal is digitally compensated by said compensation coefficient to produce a distorted digital signal, said compensation coefficient specifying a degree of nonlinear distortion of said digital input signal produced in said distorted digital signal, and in which the distorted digital signal is converted by said digital-to-analog converter to produce an analog signal having reduced nonlinear distortion.
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2. A method of operating a digital-to-analog converter, said method comprising the steps of:
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a) generating a digital calibration signal having at least one frequency component; b) operating said digital-to-analog converter in a calibration mode to convert said digital calibration signal to produce an analog signal; c) detecting, in said analog signal, a frequency component generated from said digital calibration signal by nonlinearity, to produce a compensation coefficient; and d) operating said digital-to-analog converter in a conversion mode in which a digital input signal is digitally compensated by said compensation coefficient to produce a distorted digital signal, and in which the distorted digital signal is converted by said digital-to-analog converter to produce an analog signal having reduced distortion, wherein said step (a) of generating includes generating first and second frequencies in said digital calibration signal, and wherein said frequency component generated from said digital calibration signal by nonlinearity is an intermodulation component produced by intermodulation between said first frequency and said second frequency. - View Dependent Claims (3, 4)
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5. A method of calibrating a digital-to-analog converter, said method comprising the steps of:
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a) generating a digital calibration signal having at least one frequency component; b) compensating said digital calibration signal in accordance with at least one compensation coefficient to produce a distorted version of said digital calibration signal, said compensation coefficient specifying a degree of nonlinear distortion of said to digital calibration signal produced in said distorted version of said digital calibration signal; c) operating said digital-to-analog converter to convert said distorted version of said digital calibration signal to produce an analog signal; d) detecting, in said analog signal, a frequency component absent from said digital calibration signal; and e) adjusting said compensation coefficient in response to the detected frequency component absent from said digital calibration signal so that the detected frequency component is reduced and said analog digital has reduced nonlinear distortion.
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6. A method of calibrating a digital-to-analog converter, said method comprising the steps of:
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a) generating a digital calibration signal having at least one frequency component; b) compensating said digital calibration signal in accordance with at least one compensation coefficient to produce a distorted version of said digital calibration signal; c) operating said digital-to-analog converter to convert said distorted version of said digital calibration signal to produce an analog signal;
.d) detecting, in said analog signal, a frequency component absent from said digital calibration signal; and e) adjusting said compensation coefficient in response to the detected frequency component absent from said digital calibration signal so that the detected frequency component is reduced, wherein said step (a) of generating includes generating first and second frequencies in said digital calibration signal, and wherein said frequency component absent from said digital calibration signal is an intermodulation component produced by intermodulation between said first frequency and said second frequency. - View Dependent Claims (7, 8, 9, 10)
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11. A method of calibrating a digital-to-analog converter, said method comprising the steps of:
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a) generating a digital calibration signal having at least one frequency component; b) compensating said digital calibration signal in accordance with at least one compensation coefficient to produce a distorted version of said digital calibration signal; c) operating said digital-to-analog converter to convert said distorted version of said digital calibration signal to produce an analog signal; d) detecting, in said analog signal, a frequency component absent from said digital calibration signal; and e) adjusting said compensation coefficient in response to the detected frequency component absent from said digital calibration signal so that the detected frequency component is reduced, wherein after step e), the compensation coefficient is adjusted by a specified amount and used to pre-distort a digital input signal, and the pre-distorted digital input signal is converted by said digital-to-analog converter to obtain a corresponding analog signal, so that the conversion of said digital input signal to said corresponding analog signal has a specified non-linear transfer function. - View Dependent Claims (12, 13)
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14. A method of calibrating a digital-to-analog converter, said method comprising the steps of:
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a) generating a digital calibration signal having a first frequency component and a second frequency component; b) operating said digital-to-analog converter in a calibration mode to convert said digital calibration signal to produce an analog signal; c) low-pass filtering said analog signal to produce a filtered analog signal; d) detecting, in said filtered analog signal, an intermodulation component, to produce a compensation coefficient, said intermodulation component being at a frequency that is a difference between said first frequency and said second frequency, said intermodulation component being generated from said digital calibration signal by nonlinearity, and said first frequency component and said second frequency component having been reduced in amplitude relative to said intermodulation component by said low-pass filtering; and d) operating said digital-to-analog converter in a conversion mode in which a digital input signal is digitally compensated by said compensation coefficient to produce a distorted digital signal, and in which the distorted digital signal is converted by said digital-to-analog converter to produce an analog signal having reduced distortion. - View Dependent Claims (15, 16, 17, 18, 19, 20)
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21. A method of operating a digital-to-analog converter, said method comprising the steps:
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(a) generating a first digital calibration signal having two frequency components; (b) operating said digital-to-analog converter in a calibration mode to convert said first digital calibration signal to produce a first analog signal; (c) detecting, in said first analog signal, a first intermodulation component generated from said two frequency components in said first digital calibration signal, to produce a first compensation coefficient; (d) generating a second digital calibration signal having two frequency components; (e) operating said digital-to-analog converter in a calibration mode to convert said second digital calibration signal to produce a second analog signal; (f) detecting, in said second analog signal, a second intermodulation component generated from said two frequency components in said second digital calibration signal, to produce a second compensation coefficient; and (g) operating said digital-to-analog converter in a conversion mode in which a digital input signal is digitally compensated by said compensation coefficients to produce a distorted digital signal, and in which the distorted digital signal is converted by said digital-to-analog converter to produce an analog signal having reduced distortion. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28)
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29. A method of using an analog-to-digital converter for calibrating a digital-to-analog converter to reduce offset of said digital-to-analog converter, said method comprising the steps of:
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a) operating said digital-to-analog converter to convert a digital input value of substantially zero to an analog signal; b) chopping said analog signal at a chopping frequency to produce a modulated signal; c) operating said analog-to-digital converter to digitize said modulated signal to produce a digitized signal; d) digitally detecting a component at said chopping frequency in said digitized signal; e) adjusting said digital input value to minimize the detected component at said chopping frequency in said digitized signal and produce a digital offset value; and f) using said digital offset value to offset a digital signal prior to conversion of said digital signal by said digital-to-analog converter. - View Dependent Claims (30)
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31. A digital-to-analog converter system having an analog output, said system comprising, in combination:
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a digital oscillator for generating a digital calibration signal having at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving the digital calibration signal and connected to said compensation coefficient storage for digitally compensating the digital calibration signal with said compensation coefficient to produce a compensated digital signal, said compensation coefficient specifying a degree of nonlinear distortion of said digital calibration signal produced in said compensated digital signal; a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to an analog signal, said analog output being connected to said digital-to-analog converter to receive said analog signal; a frequency-selective amplitude detector connected to said analog output for detecting a distortion component in said analog signal generated by nonlinearity from said calibration signal; and a compensation coefficient computation unit connected to said frequency-selective amplitude detector and said compensation coefficient storage for adjusting said compensation coefficient to reduce the distortion component detected by said frequency-selective amplitude detector and to reduce nonlinear distortion in said analog signal.
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32. A digital-to-analog converter system having an analog output, said system comprising, in combination:
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a digital oscillator for generating a digital calibration signal having at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving the digital calibration signal and connected to said compensation coefficient storage for digitally compensating the digital calibration signal with said compensation coefficient to produce a compensated digital signal; a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to an analog signal, said analog output being connected to said digital-to-analog converter to receive said analog signal; a frequency-selective amplitude detector connected to said analog output for detecting a distortion component in said analog signal generated by nonlinearity from said calibration signal; and a compensation coefficient computation unit connected to said frequency-selective amplitude detector and said compensation coefficient storage for adjusting said compensation coefficient to reduce the distortion component detected by said frequency-selective amplitude detector, wherein the digital oscillator is a dual-tone digital oscillator for generating two frequencies, and wherein the frequency-selective amplitude detector has a frequency response selecting an intermodulation component of said two frequencies. - View Dependent Claims (33, 34, 35, 36, 37)
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38. A digital-to-analog converter system having an analog output, said system comprising, in combination:
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a digital oscillator for generating a digital calibration signal having at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving the digital calibration signal and connected to said compensation coefficient storage for digitally compensating the digital calibration signal with said compensation coefficient to produce a compensated digital signal; a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to an analog signal, said analog output being connected to said digital-to-analog converter to receive said analog signal; a frequency-selective amplitude detector connected to said analog output for detecting a distortion component in said analog signal generated by nonlinearity from said calibration signal; a compensation coefficient computation unit connected to said frequency-selective amplitude detector and said compensation coefficient storage for adjusting said compensation coefficient to reduce the distortion component detected by said frequency-selective amplitude detector; and a digital multiplexer having a digital input for receiving a digital input signal and being connected to said digital oscillator and to said digital compensation unit for selecting either said digital calibration signal or said digital input signal and passing the selected signal to said digital compensation unit for digital compensation by said digital compensation unit prior to digital-to-analog conversion by said digital-to-analog converter.
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39. A digital-to-analog converter system having an analog output, said system comprising, in combination:
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a digital oscillator for generating a digital calibration signal having at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving the digital calibration signal and connected to said compensation coefficient storage for digitally compensating the digital calibration signal with said compensation coefficient to produce a compensated digital signal; a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to an analog signal, said analog output being connected to said digital-to-analog converter to receive said analog signal; a frequency-selective amplitude detector connected to said analog output for detecting a distortion component in said analog signal generated by nonlinearity from said calibration signal; and a compensation coefficient computation unit connected to said frequency-selective amplitude detector and said compensation coefficient storage for adjusting said compensation coefficient to reduce the distortion component detected by said frequency-selective amplitude detector, wherein said digital compensation unit is a polynomial generator for computing a polynomial having at least a second-order term and a third-order term using a second-order coefficient and a third-order coefficient obtained from said compensation coefficient storage.
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40. A digital-to-analog converter system having an analog output, said system comprising, in combination:
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a digital oscillator for generating a digital calibration signal having at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving the digital calibration signal and connected to said compensation coefficient storage for digitally compensating the digital calibration signal with said compensation coefficient to produce a compensated digital signal; a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to an analog signal, said analog output being connected to said digital-to-analog converter to receive said analog signal; a frequency-selective amplitude detector connected to said analog output for detecting a distortion component in said analog signal generated by nonlinearity from said calibration signal; and a compensation coefficient computation unit connected to said frequency-selective amplitude detector and said compensation coefficient storage for adjusting said compensation coefficient to reduce the distortion component detected by said frequency-selective amplitude detector, wherein said frequency-selective amplitude detector includes an analog-to-digital converter connected to said analog output for digitizing said analog signal to produce a digitized signal, a digital band-pass filter connected to said analog-to-digital converter for band-pass filtering said digitized signal to select said distortion component and produce a band-pass filtered signal; and
an amplitude detector connected to said band-pass filter for detecting amplitude of said distortion component in said band-pass filtered signal. - View Dependent Claims (41, 42, 43, 44, 45, 46)
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47. A digital-to-analog converter system having an analog output, said system comprising, in combination:
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a dual-tone digital oscillator for generating a series of digital calibration signals each having two frequency components; polynomial coefficient storage for storing polynomial coefficients, including at least a second-order coefficient and a third-order coefficient; a polynomial generator connected to said digital oscillator for receiving the digital calibration signals and connected to said polynomial coefficient storage for computing polynomials of said digital calibration signals to produce compensated digital signals; a digital-to-analog converter connected to said polynomial generator for converting said compensated digital signals to analog signals, said analog output being connected to said digital-to-analog converter to receive said analog signals; a continuous-time low-pass filter connected to said analog output for low-pass filtering said analog signals to provide low-pass filtered signals in which said two frequency components for each of said calibration signals are substantially reduced in amplitude in comparison to intermodulation components generated by nonlinearity from said calibration signals; an analog-to-digital converter for digitizing said low-pass filtered signals to produce digitized signals; digital processing circuits connected to said analog-to-digital converter and to said polynomial coefficient storage for detecting said intermodulation components, and adjusting said polynomial coefficients to minimize said intermodulation components. - View Dependent Claims (48, 49, 50, 51, 52, 53, 54, 55, 56)
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57. An oscillator for providing a low-distortion analog signal having at least one frequency component, said oscillator comprising, in combination:
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a digital oscillator for generating a digital signal having said at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving said digital signal and connected to said compensation coefficient storage for digitally compensating said digital signal with said compensation coefficient to produce a compensated digital signal, said compensation coefficient specifying a degree of nonlinear distortion of said digital signal produced in Said compensated digital signal; and a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to produce said low-distortion analog signal, wherein said degree of nonlinear distortion of said digital signal produced in said compensated digital signal reduces nonlinear distortion in said low-distortion analog signal. - View Dependent Claims (58)
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59. An oscillator for providing a low-distortion analog signal having at least one frequency component, said oscillator comprising, in combination:
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a digital oscillator for generating a digital signal having said at least one frequency component; compensation coefficient storage for storing a compensation coefficient; a digital compensation unit connected to said digital oscillator for receiving said digital signal and connected to said compensation coefficient storage for digitally compensating said digital signal with said compensation coefficient to produce a compensated digital signal; and a digital-to-analog converter connected to said digital compensation unit for converting said compensated digital signal to produce said low-distortion analog signal, wherein said digital signal has only two frequency components and said compensation coefficient has a value that minimizes intermodulation distortion of said two frequency components.
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Specification