Method and apparatus for oscillation recovery in a...

Details Method and apparatus for oscillation recovery in a... Method and apparatus for oscillation recovery in a...

2000-09-15

2002-03-26

Wamsley, Patrick (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C327S337000, C327S345000

Reexamination Certificate

active

06362763

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a delta-sigma modulator for use with an analog-to-digital converter. More particularly, the present invention relates to the use of a delta-sigma modulator with circuitry to detect instabilities in the modulator and to restore the modulator to a stable operating condition.
2. State of the Art
The general technique of providing analog-to-digital (“A/D”) conversion of signals is well known in the art. Generally, the sampling rate required to sample an analog signal for A/D conversion must be twice the highest frequency component being sampled. This rate is commonly known as the Nyquist rate. More recently, oversampling methods have been used to sample at a rate much higher than the Nyquist rate. An advantage of using oversampling techniques is in the precision provided by the converter. With converters operating under the Nyquist rate for sampling, a higher amount of component precision and matching is required for the conversion than with converters operating under oversampling rates.
One well-known type of oversampling A/D conversion technique uses a modulator commonly referred to as a delta-sigma modulator. In an A/D converter using a delta-sigma modulator including integrator(s), comparator(s) and a digital-to-analog converter (“DAC”) in the feedback path, a low-pass decimation filter is used following the modulator to provide necessary filtering. The analog input is modulated to a digital bit stream, typically several bits wide.
As shown in the block diagram of
FIG. 1
, a delta-sigma modulator
2
receives an input
4
and produces an output
6
. A delta-sigma modulator
2
may include one or more integration stages
8
and
10
. Feedforward paths a
1
and a
2
are provided from the outputs of each integration stage
8
and
10
to a first summing junction
12
on the non-inverting input to a comparator
14
. A feedback path
15
includes a DAC b and extends between the output
6
and a second summing junction
16
.
Typically, it is desirable in the design of a delta-sigma modulator to reduce quantization noise. Reduction of quantization noise may be achieved by the selection of a transfer function for the overall modulator that possesses high in-band gain and high out-of-band attenuation, thereby shaping the quantization noise spectrum advantageously. To appropriately shape the overall modulator transfer function, one or more additional integration stages are included within the modulator circuitry, thereby increasing the order of the modulator.
Despite the advantages of higher order modulators, they are well known to be only conditionally stable. All high order modulators become unstable for inputs that exceed certain bounds. Instability may also occur after power-on since operational amplifier (“op-amp”) integrators with arbitrary initial states may place the modulator in an unstable region of its state space. In the case of a large input exceeding a stability threshold, even when the input is brought back below the stability threshold, oscillation may still persist. Therefore, a means for detecting instability and restoring the loop back to a stable condition is necessary in higher order modulators.
One approach to correcting the instability found in higher order modulators (three or more integration stages) is to use state-variable clamping techniques.
FIG. 2
shows an integration stage
18
of a modulator including an op-amp
20
having an integration capacitor
22
and a limiter
24
coupled between the non-inverting input and the output of the op-amp
20
. A non-linear element, such as a limiter, coupled across the integrating capacitor
22
prevents large values from appearing at the integrator output. Typically, for a higher order modulator circuit, the non-linear elements are set to turn “ON” at about 20-50% higher than the peak-to-peak integrator swings. One example of a limiting scheme implemented in an integrator stage is shown in U.S. Pat. No. 5,977,895 to Murota et al. (Nov. 2, 1999), entitled “WAVEFORM SHAPING CIRCUIT FOR FUNCTION CIRCUIT AND HIGH ORDER DELTA SIGMA MODULATOR.” For the approach shown in Murota et al., however, the input signal at each stage must be limited to a few hundred millivolts to maintain stability. As a result, the first stage integrator capacitor tends to be very large relative to the input capacitor. In a high performance A/D modulator design where input capacitors are relatively large, the addition of an even larger first stage integration capacitor results in an area- and power-intensive circuit.
Another approach to overcoming loop stability problems in higher order modulators is to detect an overload condition and degrade the performance of the modulator by changing the modulator integrator coefficients of operation. An example of this approach is disclosed in U.S. Pat. No. 6,064,326 to Krone et al. (May 16, 2000), entitled “ANALOG-TO-DIGITAL CONVERSION OVERLOAD DETECTION AND SUPRESSION.” Although this approach apparently does not destroy the information stored on the integrators, it does require additional switched capacitors and switch branches to implement.
Yet another approach to solving instability problems in higher order modulators is to sense instability and to responsively reset the circuit to a known state. There are two conventional methods of sensing instability: 1) Looking for integrator input values above a certain value using a comparator; and 2) Looking for long strings of 1s or 0s in the digital bit stream.
U.S. Pat. No. 5,012,244 to Wellard et al. (Apr. 30, 1991), entitled “DELTA-SIGMA MODULATOR WITH OSCILLATION DETECT AND RESET CIRCUIT,” includes an example of the first method of sensing instability for higher order modulators.
FIG. 3
, herein, is similar to that shown in
FIG. 1
of the Wellard et al. patent and shows a circuit in which a fourth order delta-sigma modulator
30
uses an oscillation detect comparator
32
to detect instability in the signal at the output of the second integrator
36
. If instability is detected, the oscillation detect comparator
32
resets the circuit by short-circuiting the outputs of each of the four integrators
34
,
36
,
38
and
40
with its respective input by closing a switch
42
,
44
,
46
and
48
coupled across each of the integrators
34
,
36
,
38
and
40
.
FIG. 4
includes a more detailed diagram of a single integrator stage
50
of the prior art delta-sigma modulator circuit of FIG.
3
. Each stage
50
includes an integrating capacitor
52
and a switch
54
coupled in parallel between the output
56
and an input
58
of an op-amp
60
. The op-amp
60
shown in
FIG. 4
is configured as a single-ended structure, meaning it has only a single output, rather than as a differential-ended structure, meaning it has two outputs. Both singular and differential structures for op-amps are well known in the art.
For the modulator circuit shown in FIG.
3
and the integrator shown in
FIG. 4
, resetting the integrators
34
,
36
,
38
and
40
by closing the switches
42
,
44
,
46
and
48
returns all four integrators
34
,
36
,
38
and
40
to known, stable states. However, resetting the integrators
34
,
36
,
38
and
40
in this manner also eliminates any common mode information stored between the inputs and outputs of the integrators
34
,
36
,
38
and
40
by shorting the integrating capacitors.
Therefore, there is a need for an A/D modulator circuit which overcomes the stability problems experienced by higher order modulator circuits of the prior art, without losing all of the information stored by the modulator integrator stages.
SUMMARY OF THE INVENTION
It is an object of the invention to have an A/D modulator circuit which overcomes the stability problems experienced by prior art higher order modulator circuits.
It is another object of the invention to have a high order delta-sigma modulator circuit which is capable of retaining stored common mode information throughout a restore operation.
The present invention provides a higher order (three or mor

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