sigma-delta : unintuitive

"Randy Yates" <yates@ieee.org> wrote in message
news:3F272526.DC84F022@ieee.org...
> Hi Matt,
>
> I wasn't clear when I stated "digital signal." What I meant was a signal
> that is digitized (or re-digitized) using a simple N-bit quantizer. The
> noise is about 6*N dB below the full-scale signal.

You were clearer than I was, I think.  My concern is that this only holds as
long as the input is inside the full scale range of the quantizer.  In the
S-D modulator, the quantizer is quantizing the feedback, and I can't find a
mechanism that limits the feedback to that range, even if the input signal
is guaranteed to be so limited.

> I believe what you are talking about is the stability of the modulator
> due to the feedback loop.

Right, but stability isn't quite sufficient.  Certain inputs may be
amplified by the quantizer enough to drive the feedback outside of the
quantizer's input range.  At that point, the quantization noise increases
with the feedback magnitude, or, more likely, the feedback hits a supply
rail and the analysis goes out the window.

> That's a whole other subject and one that is
> quite complex from what I've gathered. I think they've analyzed first-
> and second-order loops analytically for stability, but higher-order
> loops are still a shot in the dark (I think). I have some material on
> analyzing stability at work - I'd be happy to provide the references when
> I get in tomorrow if you're interested.

Yes, please ;-)

kbc32@yahoo.com (kbc) wrote in message news:<a382521e.0307280611.7dbc8958@posting.google.com>...
> I am finding the sigma-delta adc difficult to understand.
> 
> The only way, i feel, to reduce the quantization noise ,
> given the bit-resolution , is to sample non-uniformly.
> 
> Is that what happens ?
> 
>        shankar


No, that's not what happens.

What one does is to oversample by a lot, and use noise-shaping
filters. This means that you can move the quantization noise way up in
frequency, where it doesn't matter, and then take it out with a simple
filter.

Look in Jayant and Noll at "A*PCM".

"Matt Timmermans" <mt0000@sympatico.nospam-remove.ca> wrote in message news:<O3PVa.4088$537.656342@news20.bellglobal.com>...
> "Randy Yates" <yates@ieee.org> wrote in message
> news:3F272526.DC84F022@ieee.org...
> > I have some material on
> > analyzing stability at work - I'd be happy to provide the references when
> > I get in tomorrow if you're interested.
> 
> Yes, please ;-)

Okie dokie, Matt. Here is a list of articles I've collected on delta
sigma conversion in general, not just stability. I have the electronic
forms so if you email me at randy.yates@sonyericsson.com I can attach
and send them to you:

1. "Stability Analysis of the Second Order Sigma Delta Modulator," Philip
Steiner and Woodward Yang, Division of Applied Sciences, Harvard;

2. "A Higher Order Topology for Interpolative Modulators for Oversampling
A/D Converters," Chao, Nadeem, Lee, and Sodini, IEEE Transactions on Circuits
and Systems, vol. 37, no. 3, March 1990.

3. "An Empirical Study of High-Order Single-Bit Delta-Sigma Modulators," 
Richard Schreier, IEEE Transactions on Circuits and Systems, vol. 40, no. 8,
August 1993.

4. "Effective Dithering of Sigma-Delta Modulators," Steven R. Norsworthy,
AT&T Bell Laboratories, 

5. "A Comparison of Dithered and Chaotic Sigma-Delta Modulators," Chris
Dunn and Mark Sandler, JAES, April 1996.

6. "Psychoacoustically Optimal Sigma Delta Modulation," Chris Dunn and
Mark Sandler, JAES, April 1997.