US 5,594,612 A · Analog-to-digital converter with digital linearity correction

Abstract

A analog calibration signal including at least one frequency component is generated by a very pure signal source, such as a digital oscillator and a digital-to-analog converter (DAC) that has been calibrated to be ultralinear. The analog calibration signal is converted by an analog-to-digital converter (ADC) to a digital signal. The digital signal is digitally compensated in accordance with compensation coefficients to produce a compensated digital signal. The compensated digital signal is digitally processed to isolate and measure distortion components, and the compensation coefficients are adjusted in response to the distortion components in order to reduce the distortion components. Feedback causes the distortion components to be minimized so that the compensation coefficients correct the nonlinearity in the analog-to-digital converter. Preferably the analog calibration signal is a two-tone signal, the digital signal is compensated by computing a polynomial, the compensation coefficients are coefficients of the polynomial, and the distortion components are intermodulation components. After calibration, the calibration coefficients can be adjusted to provide a specified non-linear transfer function. The method can also be used to compensate for nonlinearity in analog signal conditioning circuits that condition the analog signal prior to conversion. Moreover, the calibrated ADC could be used to re-calibrate the DAC, which could be used to re-calibrate the ADC, for enhanced linearity.

Claims60

Dependent claims

A method of digitally compensating a digital signal produced by an analog-to-digital converter to reduce nonlinear distortion, said method comprising the steps of:

a) generating an analog calibration signal having at least one frequency component;
b) applying said analog calibration signal to said analog-to-digital converter to produce said digital signal by conversion of said analog calibration signal;
c) compensating said digital signal in accordance with at least one compensation coefficient to produce a compensated digital signal;
d) digitally processing said compensated digital signal to select a distortion component in said compensated digital signal produced by nonlinearity from said frequency component and to obtain a measurement of the selected distortion component;
and
e) adjusting said compensation coefficient in response to said measurement for reducing said distortion component;
wherein said step (a) of generating includes generating a first frequency and a second frequency in said analog calibration signal, and wherein said distortion component is an intermodulation component produced by intermodulation between said first frequency and said second frequency.

4 more elements

A method of calibrating an analog-to-digital converter for reduced nonlinear distortion, said method comprising the steps of:

a) generating an analog calibration signal having a first frequency component at a first frequency and a second frequency component at a second frequency;
b) operating said analog-to-digital converter in a calibration mode to convert said analog calibration signal to produce a digital signal;
c) digitally filtering said digital signal to produce a filtered digital signal;
d) detecting, in said filtered digital 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
e) analog calibration signal by nonlinearity;
and operating said analog-to-digital converter in a conversion mode in which an analog input signal is converted to a corresponding digital signal by said analog-to-digital converter, and said corresponding digital signal is digitally compensated by said compensation coefficient to produce a compensated digital signal having reduced nonlinear distortion.

3 more elements

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

a) generating an analog calibration signal having a first frequency component and a second frequency component;
b) operating said analog-to-digital converter in a calibration mode to convert said analog calibration signal to produce a digital signal;
c) digitally filtering said digital signal to produce a filtered digital signal;
d) detecting, in said filtered digital 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 analog calibration signal by nonlinearity;
and
e) operating said analog-to-digital converter in a conversion mode in which an analog input signal is converted to a corresponding digital signal by said analog-to-digital converter, and said corresponding digital signal is digitally compensated by said compensation coefficient to produce a compensated digital signal having reduced distortion;
wherein said step c) of digitally filtering said digital signal to produce a filtered digital signal includes passing said digital signal through a digital band-pass filter to select said intermodulation component;
wherein said detecting said intermodulation component includes detecting absolute value of said intermodulation component passing through said digital band-bass filter;
and
wherein the method further includes iteratively adjusting said compensation coefficient by either increasing or decreasing said compensation coefficient by an amount proportional to said absolute value, and determining whether the compensation coefficient is to be increased or decreased depending on whether the compensation coefficient was increased or decreased during a last iteration, and depending on whether a last change to the compensation coefficient caused the absolute value to increase or decrease.

7 more elements

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

(a) generating a first analog calibration signal having two frequency components;
(b) operating said analog-to-digital converter in a calibration mode to convert said first analog calibration signal to produce a first digital signal;
(c) detecting, in said first digital signal, a first intermodulation component generated from said two frequency components in said first analog calibration signal, to produce a first compensation coefficient;
(d) generating a second analog calibration signal having two frequency components;
(e) operating said analog-to-digital converter in a calibration mode to convert said second analog calibration signal to produce a second digital signal;
(f) detecting, in said second digital signal, a second intermodulation component generated from said two frequency components in said second analog calibration signal, to produce a second compensation coefficient;
and
(g) operating said analog-to-digital converter in a conversion mode in which an analog input signal is converted by said analog-to-digital converter to produce a corresponding digital signal, and the corresponding digital signal is digitally compensated by said first and second compensation coefficients to produce a digital output signal having reduced distortion.

5 more elements

A method of calibrating an analog-to-digital converter for reduced nonlinear distortion, said method comprising the steps of:

a) calibrating a digital-to-analog converter for reduced nonlinear distortion;
b) generating a digital calibration signal having at least one frequency component;
c) using the calibrated digital-to-analog converter to convert said digital calibration signal to an analog calibration signal;
applying said analog calibration signal to said analog-to-digital converter to convert said analog calibration signal to a corresponding digital signal;
e) digitally processing said corresponding digital signal to generate a compensation coefficient for reducing nonlinear distortion in said corresponding digital signal;
and
f) operating said analog-to-digital converter in a conversion mode in which said analog-to-digital converter converts an analog input signal to produce a digitized signal, and in which the digitized signal is digitally compensated by said compensation coefficient to produce a compensated digital signal having reduced nonlinear distortion.

4 more elements

An analog-to-digital converter system having a digital output, said system comprising, in combination:

a calibration oscillator for generating an analog calibration signal having at least one frequency component;
an analog-to-digital converter connected to said calibration oscillator for converting said analog calibration signal to a digital signal;
compensation coefficient storage for storing a compensation coefficient;
a digital compensation unit connected to said analog-to-digital converter for receiving the digital signal and connected to said compensation coefficient storage for digitally compensating the digital signal with said compensation coefficient to produce a compensated digital signal on said digital output;
a frequency-selective amplitude detector connected to said digital output for detecting a distortion component in said compensated digital signal generated by nonlinearity from said analog 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 oscillator is a dual-tone 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

An analog-to-digital converter system having a digital output, said system comprising, in combination:

a calibration oscillator for generating an analog calibration signal having at least one frequency component;
an analog-to-digital converter connected to said calibration oscillator for converting said analog calibration signal to a digital signal;
compensation coefficient storage for storing a compensation coefficient;
a digital compensation unit connected to said analog-to-digital converter for receiving the digital signal and connected to said compensation coefficient storage for digitally compensating the digital signal with said compensation coefficient to produce a compensated digital signal on said digital output;
a frequency-selective amplitude detector connected to said digital output for detecting a distortion component in said compensated digital signal generated by nonlinearity from said analog 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 a digital band-pass filter connected to said digital compensation unit for band-pass filtering said compensated digital 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;
and
wherein said amplitude detector is an asynchronous detector detecting a magnitude of said distortion component, and wherein said compensation coefficient computation unit includes means for iteratively adjusting said compensation coefficient and means for determining whether the compensation coefficient is to be increased or decreased depending on whether the compensation coefficient was increased or decreased during a last iteration, and depending on whether a last change to the compensation coefficient caused said magnitude to increase or decrease.

8 more elements

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

a dual-tone digital oscillator for generating a series of digital calibration signals each having two frequency components;
a digital compensation unit connected to said dual-tone digital oscillator for distorting said digital calibration signals to generate distorted digital calibration signals;
a digital-to-analog converter connected to said digital oscillator for converting said distorted digital calibration signals to produce analog calibration signals having reduced distortion;
an analog-to-digital converter connected to said digital-to-analog converter for converting said analog calibration signals to corresponding digital signals;
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 analog-to-digital converter for receiving the corresponding digital signals, said polynomial generator being connected to said polynomial coefficient generator for computing polynomials of said corresponding digital signals to produce compensated digital signals at a digital output;
and
digital processing circuits connected to said polynomial generator for detecting intermodulation components of said two frequency components in said compensated digital signals, and said digital processing circuits being connected to said polynomial coefficient storage adjusting said polynomial coefficients to minimize said intermodulation components.

5 more elements

Dependencies are inferred from the claim text.

Analog-to-digital converter with digital linearity correction

Life of this patent

Analog-to-digital converter with digital linearity correction

Life of this patent

Abstract

Claims60