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Digital-to-analog converter with digital linearity correction
| Details |
Inventors: Henrion, W. S.; Kerth, Donald A.;
Assignee: Crystal Semiconductor Corporation (Austin, TX)
Primary Examiner: Hoff; Marc S.
Assistant Examiner:
Attorney, Agent or Firm: Arnold, White & Durkee
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. |
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DETAILED DESCRIPTION In accordance with a basic aspect of the present invention, there is provided a method of operating a digital-to-analog converter.
The method includes the steps of: generating a digital calibration signal having at least one frequency component; operating the digital-to-analog converter in a calibration mode to convert the digital calibration signal to produce an analog signal; detecting, in the analog signal, a frequency component generated from the digital calibration signal by nonlinearity, to produce a compensation coefficient; and operating the digital-to-analog converter in a conversion mode in which a digital input signal is digitally compensated by the compensation coefficient to produce a distorted digital signal, the compensation coefficient specifying a degree of nonlinear distortion of the digital input signal produced in the distorted digital signal; and in which the distorted digital signal is converted by the digital-to-analog converter to produce an analog signal having reduced nonlinear distortion.
This method has the advantage that a linearity reference is not needed, because the calibration signal having at least one frequency component can be precisely generated using digital techniques.
This method also has the advantage that it is not limited to a particular kind of converter technology.
In accordance with another aspect, the present invention provides a method of calibrating a digital-to-analog converter.
This method includes the steps of: generating a digital calibration signal having at least one frequency component; compensating the digital calibration signal in accordance with at least one compensation coefficient to produce a distorted version of the digital calibration signal, the compensation coefficient specifying a degree of nonlinear distortion of the digital calibration signal produced in the distorted version of the digital calibration signal; operating the digital-to-analog converter to convert the distorted version of the digital calibration signal to produce an analog signal; detecting, in the analog signal, a frequency component absent from the digital calibration signal; and adjusting the compensation coefficient in response to the detected frequency component absent from the digital calibration signal so that the detected frequency component is reduced and the analog signal has reduced nonlinear distortion
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