Method and apparatus for providing signal dependent dither generator for sigma-delta modulator

A method to operate a sigma-delta modulator of a type that includes a loop filter followed by a quantizer, comprising steps of:

• sampling an amplitude of an input signal to the loop filter;
• and
• generating a dither current signal for summation with a quantizer current signal, said dither current signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the sampled amplitude of the input signal,
• wherein the step of generating operates at least one linear feedback shift register to control the on and off states of a plurality of current sources forming a DAC and to control the amplitude of the dither current signal, and
• wherein the step of operating the at least one linear feedback shift register operates a linear feedback shift register having a plurality of taps for implementing a FIR filter to shape the spectrum of the dither current signal.

A method to operate a sigma-delta modulator of a type that includes a loop filter followed by a quantizer, comprising steps of:

• sampling an amplitude of an input signal to the loop filter;
• and
• generating a dither current signal for summation with a quantizer current signal, said dither current signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the sampled amplitude of the input signal,
• wherein the step of sampling samples the input signal for dither signal amplitude control out of phase with a sampling of the input signal by the sigma-delta modulator.

A method to operate a sigma-delta modulator of a type that includes a loop filter followed by a quantizer, comprising steps of:

• sampling an amplitude of an input signal to the loop filter;
• and
• generating a dither current signal for summation with a quantizer current signal, said dither current signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the sampled amplitude of the input signal,
• wherein the step of sampling samples the input signal for dither signal amplitude control by operating a plurality of window detectors in parallel, and wherein the dither current signal is amplitude modulated in a plurality of predetermined quantized steps that are a function of the number of window detectors.

A method to operate a sigma-delta modulator of a type that includes a loop filter followed by a quantizer, comprising steps of:

• sampling an amplitude of an input signal to the loop filter;
• and
• generating a dither current signal for summation with a quantizer current signal, said dither current signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the sampled amplitude of the input signal,
• wherein the step of sampling samples the input signal for dither signal amplitude control by generating a current having a value that represents a square of the magnitude of the sampled input signal, subtracts the current from a reference current to obtain a difference current, and amplitude modulates the dither current signal using the difference current.

A sigma-delta modulator of a type that includes a loop filter followed by a quantizer, comprising:

• circuitry for sampling an amplitude of an input signal to the loop filter;
• and
• circuitry for generating a dither current signal for summation with a quantizer current signal, said dither current signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the sampled amplitude of the input signal,
• wherein said dither current signal generating circuitry comprises at least one linear feedback shift register to control the on and off states of a plurality of current sources forming a DAC and the amplitude of the dither current signal, and
• wherein said dither current signal generating circuitry comprises a linear feedback shift register having a plurality of taps for implementing a FIR filter to shape the spectrum of the dither current signal.

A method for operating a sigma-delta modulator having a quantizer, comprising steps of:

• inputting a signal to the sigma-delta modulator;
• and
• generating a dither signal for injection into a quantizer input signal, said dither signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the amplitude of the input signal to the sigma-delta modulator;
• wherein
• said step of generating includes a step of shaping a spectrum of the dither signal so as to reduce in-band noise.

A sigma-delta modulator, comprising

• a quantizer;
• an input node for inputting a signal to the sigma-delta modulator;
• and
• circuitry for generating a dither signal for combination with a quantizer input signal, said dither signal being generated to have a pseudorandom amplitude that is modulated so as to be inversely proportional to the amplitude of the input signal to the sigma-delta modulator, said circuitry operating to shape a spectrum of the dither signal so as to reduce in-band noise.