Stabilization of single loop sigma-delta modulators of a...

Details Stabilization of single loop sigma-delta modulators of a... Stabilization of single loop sigma-delta modulators of a...

1999-03-29

2001-01-02

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C341S143000, C330S20700P

Utility Patent

active

06169507

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of digital-to-analog and analog-to-digital signal converters, and, more particularly, to a sigma-delta modulator used in such converters.
BACKGROUND OF THE INVENTION
Sigma-delta modulators used in digital/analog and analog/digital converters, and particularly those of high order (≦2) are likely to have instability problems which should be accurately checked. A sigma-delta modulator may be represented as in FIG.
1
. The function of such a structure is that of reducing the precision of the digital input signal (IN) without changing the quality of the signal. This is done by using a quantizer that eliminates some of the precision bits while a feedback block keeps the noise introduced by the quantizer in a frequency band different from the frequency band of the signal.
In an extreme case, the quantizer reduces the signal to one bit, so that the function may be implemented with a simple comparator. A thorough analysis of the performance of the system shown in
FIG. 1
cannot be done by using the normal techniques that are used for analyzing linear networks because of the presence of a nonlinear block, i.e., the quantizer. However, it is possible to linearly operate the modulator to obtain a model capable of providing qualitative indications of the system's behavior. To do this, the quantizer is modeled as an external source of noise “q” (i.e., white noise), thus eliminating any nonlinear block from the system, and thereafter, the output signal contribution of the input signal and of the added noise. The equivalent scheme becomes that shown in FIG.
2
.
By solving the block diagram of
FIG. 2
, the following expression is obtained: Out(z)=In(Z)*[A(Z)/1−B(Z)]+q(Z)*[1/1−B(Z)]. The output is made up of the input signal filtered by the function [A(Z)/1−B(Z)] and of the noise filtered by the function [1/1−B(Z)]. By appropriately defining the two functions A(Z) and B(Z), it is possible to obtain the desired functions for both the input signal and for the noise.
To keep the signal and the noise separated in frequency, A(Z) and B(Z) are chosen so that the system operates as a low-pass filter for the signal and as a high-pass filter for the quantization noise. The performance of sigma-delta modulators increase with the order of the A(Z) and B(Z) functions, but problems arise for ensuring closed loop stability for orders ≦22. The problem is more subtle than it may appear because the quantizer is a nonlinear block. In particular, it is even possible for the comparator to provide a certain gain. Such a gain primarily depends on the input signal amplitude. If the signal amplitude increases, this fictitious gain of the comparator decreases.
The stability of a feedback system is significantly conditioned by the open loop gain. For a certain interval of gain values, the stability is maintained while outside this interval the system becomes unstable. Usually, in the case of sigma-delta modulators, the system is such that excessively low gains introduce a state of instability. Consequently, an irreversible phenomena takes place. If for any reason the signal upstream of the comparator increases excessively, the gain of the comparator decreases. This in turn reduces the loop gain and the system becomes unstable. In such a state of instability, the internal signals increase and the system is prevented from regaining stability.
This type of phenomenon is very common in sigma-delta modulators, and to prevent this problem, there are different techniques ranging in varying complexity. The problem of instability arises rarely and only under particular conditions—typically at switch-on, and in the case of low frequency signals having large amplitudes. However, it is very important to return the system to a correct operation. Otherwise, the system will remain unstable even after the triggering conditions have ended and the sigma-delta modulator would stop functioning at the first critical signal.
Traditional approaches addressing these problems are based on the principle of recognizing the state of instability, and accordingly, intervening on the internal states to bring the system back to an operating point within the stability zone. These methods differ among each other on how the state of instability is detected and in the way the system is brought back to a correct operation.
These known approaches range from a simpler system that intervenes only when a register overflows by zeroing all the internal states, to more complex systems wherein critical situations are recognized before they degenerate into a real instability. Therefore, an intervention is limited to just an appropriate point without major consequences on the system as a whole.
SUMMARY OF THE INVENTION
A sigma-delta modulator in accordance with the invention becomes unconditionally stable using an internal modification rather than the addition of dedicated external control circuits. In operation, the sigma-delta modulator never exits the stability zone irrespective of the amplitude of the input signal. The comparator is required to provide as output a one bit signal for converting into an analog signal by the use of a simple electronic switch. Therefore, there is no need to have an output signal having a larger number of bits because this would require a more complex analog stage connected in cascade.
In contrast, with respect to the feedback, a multi-bit signal may be acceptable for the conventional architecture without creating substantial complications. Fundamentally, the sigma-delta modulater is based upon the use of two distinct comparators. A conventional one-bit comparator is dedicated for generating the output signal, and a second or auxiliary multilevel comparator is dedicated for generating a feedback signal. In this way, the signal path through the sigma-delta modulator splits upstream of the quantizer, allowing for a diversification of the feedback signal from the output signal.


REFERENCES:
patent: 4987416 (1991-01-01), Leslie
patent: 5187482 (1993-02-01), Tiemann et al.
patent: 5208594 (1993-05-01), Yamazaki
patent: 5777512 (1998-07-01), Tripathi et al.
patent: 95 34955 (1995-12-01), None

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