Sigma-delta analog/digital converter with adaptive dither

Details Sigma-delta analog/digital converter with adaptive dither Sigma-delta analog/digital converter with adaptive dither

2000-07-13

2001-11-20

JeanPierre, Peguy (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C341S131000

Reexamination Certificate

active

06320526

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to signal processing systems and, more particularly, to sigma-delta (&Sgr;&Dgr;) analog/digital converters.
BACKGROUND OF THE INVENTION
&Sgr;&Dgr; analog/digital converters are commonly used in modern signal processing systems for converting analog input signals into digital signals, such as, for example, in audio applications and video systems. A typical drawback of &Sgr;&Dgr; converters is the phenomenon of intrinsic generation of tones that may be reflected back in the band of interest, for example the audio band, in presence of low dynamic input signals. Particularly undesirable are the tones near half a value of the sampling frequency because they have the greatest energetic content and are also more likely to be reflected back in the audio band as a consequence of nonlinearities and intermodulation effects.
Dithering techniques are commonly used to curb these undesirable phenomena. A dither signal, injected in the modulator that converts the analog signal into digital, has the function of making the error of the comparator of the analog/digital converter, often referred to as the modulator, as independent as possible from the input signal. Thus, the generation of tones and the resulting speed of their energy on the whole band may be averted.
Dithering techniques normally used in &Sgr;&Dgr; analog/digital converters are based on the injection of deterministic signals, whose energy is spectrally distributed outside the band of interest. The negative effect of dithering techniques manifests itself when the input signals are near the maximum design dynamic. The energy of the dither signal summed to that of the input signal to be converted may bring the stages of the modulator to operate under saturation conditions. In order to avert this problem, the dither signal may be attenuated or even deactivated in the presence of high dynamic input signals, implementing what is commonly referred to as an adaptive dither.
In the case of audio band converters, the most common signals being used are relatively continuous or sinusoidal signals of a frequency sufficiently above the band of the analog input signals being subject to analog/digital conversion. Continuous dither signals have a scarce effect, while the use of sinusoidal signals require a considerably more complex circuitry. Dither signals with a spectrum similar to white noise, notably very efficient for eliminating the tones, may be generated only digitally and for this reason are generally employed in digital/analog &Sgr;&Dgr; converters. A dithering signal, with a spectrum similar to that of white noise, has never been applied to analog/digital &Sgr;&Dgr; converters.
SUMMARY OF THE INVENTION
An object of this invention is to provide an analog/digital &Sgr;&Dgr; converter with an effective adaptive dither using a dithering signal with a spectrum similar to white noise. Fundamentally, the analog/digital &Sgr;&Dgr; converter of the invention has an input into which is fed an analog input signal, and an output producing a digital signal which represents the converted analog signal. Also, the converter includes a circuital for generating a dithering signal with an amplitude adaptively regulated as a function of the amplitude of the analog input signal as sensed by a comparator whose output effects the regulation. The converter of the invention uses a dithering signal that is digitally generated and which is thereafter converted into an analog noise having a spectrum substantially similar to that of white noise.
Indeed, to obtain a sensible reduction of spurious tones in the output signal spectrum, the dither signal must have a nonnegligible power compared to the maximum energy of the input signal. Filtering permits attenuation of the power level of the dither signal within the frequency band of interest, thus preserving the signal
oise ratio within acceptable limits. A second-order high-pass filtering in the digital domain may be implemented upstream of the converter or it may be implemented in the analog domain downstream of the digital/analog converter. The realization of a second-order high-pass digital filter requires a digital/analog converter with a relatively high number of bits. Conversely, the realization of a second-order analog filter implies a nonnegligible circuit complexity.
Most preferably, the filtering is carried out by using two first-order high-pass filters, a first filter realized in the digital domain upstream of the digital/analog converter and a second first-order high-pass filter coupled in cascade to the digital/analog converter in the analog domain of the dither signal. In this manner, the required filtering of the dither signal is implemented while conveniently limiting the number of bits of the digital/analog modulator (digital signal already pre-filtered) and of a first-order analog filter of considerably reduced complexity compared to that of a second-order filter is employed in cascade to the modulator.
According to a preferred embodiment, the circuit components needed for practicing the invention include a digital white noise generator, at least a digital divider of the signal produced by the generator coupled to the output of the generator, and a path selector having an input coupled to the output of the generator. Also included is at least a second input coupled to the output of the digital divider, controlled by the comparator of the analog input signal, a digital high-pass filter coupled in cascade to the path selector, and a digital/analog converter coupled in cascade to the digital filter. Furthermore, a high-pass analog filter is coupled in cascade to the digital/analog converter of the dither signal and whose output is coupled to a node of injection of the dithering signal of the &Sgr;&Dgr; analog/digital converter.


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