Dither signal insertion inversely proportional to signal level in delta-sigma modulators

DETAILED DESCRIPTION The foregoing and other problems are overcome and the foregoing objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
The teachings of this invention provide embodiments of low complexity, single-bit SD modulators that employ a dither signal having an amplitude that is a function of the amplitude of the input signal to the SD modulator.
The teachings of this invention apply as well to multi-bit SD modulators.
In these embodiments pseudorandom noise is added to an input of a SD modulator quantizer as a dither signal, and the amplitude of the pseudorandom noise is controlled in such a manner as to be inversely proportional to the amplitude of the input signal, i.
e.
, the amplitude of the dither signal is smallest when the amplitude of the input signal is largest and vice versa.
In the presently preferred embodiments at least one linear feedback shift register (LFSR) is used to generate a pseudorandom code sequence that in turn is used to control the switching of voltage potentials to inputs of banks of voltage variable capacitances coupled to input nodes of the quantizer.
The variation in capacitance at the input nodes of the quantizer generates voltage transitions in the quantizer input signal, thereby dithering the input signal to the quantizer.
In a further embodiment of this invention embodiment of this invention the instantaneous amplitude of the input signal is quantized with at least one low complexity window detector which controls the clock signal to the at least one LFSR.
A method is disclosed to operate a sigma-delta modulator of a type that includes a quantizer.
The method has steps of (a) sampling an amplitude of an input signal to the sigma-delta modulator; and (b) controlling the switching of a capacitance bank in accordance with the sampled amplitude of the input signal for generating a dither signal at an input of the quantizer.
The dither signal is generated to have a pseudorandom amplitude that is inversely proportional to the sampled amplitude of the input signal