US 5,583,501 A · Digital-to-analog converter with digital linearity correction

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.

Claims59

Dependent claims

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.

2 more elements

A method of calibrating a digital-to-analog converter, said method comprising the steps of:

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.

3 more elements

A method of operating a digital-to-analog converter, said method comprising the steps:

(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.

5 more elements

A digital-to-analog converter system having an analog output, said system comprising, in combination:

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.

5 more elements

A digital-to-analog converter system having an analog output, said system comprising, in combination:

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.

6 more elements

A digital-to-analog converter system having an analog output, said system comprising, in combination:

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.

4 more elements

Dependencies are inferred from the claim text.

Digital-to-analog converter with digital linearity correction

Life of this patent

Digital-to-analog converter with digital linearity correction

Life of this patent

Abstract

Claims59