Delta-sigma ADC question

I read in sci.electronics.design that Randy Yates 
<randy.yates@sonyericsson.com> wrote (in 
<xxpfyxwb6cn.fsf@usrts005.corpusers.net>) about 'Delta-sigma ADC 
question', on Tue, 12 Apr 2005:

> Back in my old school/analog days, I was taught that settling time is 
>inversely proportional to bandwidth.

That applies to minimum-phase networks. Anything with a delay in it is 
in principle not minimum-phase and there is no general relation between 
settling time and bandwidth.
-- 
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I read in sci.electronics.design that John Larkin 
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote (in 
<8j6o51t1hn361rf2usqr5bvtte41dhfmrp@4ax.com>) about 'Delta-sigma ADC 
question', on Tue, 12 Apr 2005:
>I've never understood that. You'll have to explain it to me some day.

Look for Audio Engineering Society papers by S Lipshitz and J 
Vanderkooy. If you are like me, you still won't understand it. (;-)
-- 
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Andre <no_spam@fischer-zoth.de> wrote in news:d3gkoo$57c$02$1@news.t-
online.com:

> Additionally, you might have to deal with delays. Typical delay times 
> between analog input and digital output of delta-sigma ADs are around 
15 
> samples! This might make synching mux timing and sampled data hard to 
do.
>

This delay is exactly why SAR converters are used in feedback systems.

A very big advantage to sigma delta converters is that the analog 
antialiasing filters are trivial since the actual sample rate is much 
high than the "effective" sample rate.

The internal digital filters may have stopband attenuation of 90dB at 
something near 1/2 the "sample rate" This of course is a major reason for 
the added delay. The better converters have better filters which have 
more delay.

You may find that low cost sigma delta converters are still a better 
choice than a muxed SAR. You can find 4 channel devices that smaple at 
100kHz or 200kHz for reasonable cost. 


-- 
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

John Larkin wrote:
> On Tue, 12 Apr 2005 10:33:59 -0500, Rob Gaddi
> <rgaddi@bcm.YUMMYSPAMtmc.edu> wrote:
> 
> 
>>Ville Voipio wrote:
>>
>>>Actually, it would be more precise to talk about "oversampling"
>>>converters, because that's what makes the difference. Not the
>>>actual converter topology or modulator order.
>>>
>>>- Ville
>>>
>>
>>Not exactly.  The pole in the feedback loop of a delta-sigma converter 
>>serves to take the nominally white quantization noise power and blue 
>>shift it into the higher frequencies, which if your mixed signal system 
>>is designed correctly will then be out of band from your signal, such 
>>that the quantization noise can be filtered and decimated out.  This is 
>>in contrast to just taking say an SAR, oversampling by N, filtering, and 
>>decimating, which doesn't perform this noise shaping because it doesn't 
>>have the feedback.
> 
> 
> 
> I've never understood that. You'll have to explain it to me some day.
> 
> John
> 

the simple way:

If you model the quantization as as adding white noise with a power
proportional to the number of bits, you'll see that for a delta-sigma
modulator that noise will show up at the output high-passed, the power
is still the same but "pushed" towards higher frequencies.
When you then lowpass and decimate you remove the higher frequencies
and thus much of the the noise.

quatization noise before lowpass and decimate:

^
|     - -
|    /
|   /
|  /
+==------+> f
          fs/2
quatization noise after lowpass and decimate:

^
|
|
|
|
+==+------> f
    fs/2

With a SAR the noise is flat at the output, so lowpass and decimate
will remove less of the noise power.


quatization noise before lowpass and decimate:

^
|
|
|
|---------
+---------+>
           fs/2

quatization noise after lowpass and decimate:

^
|
|
|
|--
+--+-----> f
    fs/2


-Lasse

"Al Clark" <dsp@danvillesignal.com> wrote in message
news:Xns9636B5ED48E0Baclarkdanvillesignal@66.133.129.71...
> Andre <no_spam@fischer-zoth.de> wrote in news:d3gkoo$57c$02$1@news.t-
> online.com:
>
> > Additionally, you might have to deal with delays. Typical delay times
> > between analog input and digital output of delta-sigma ADs are around
> 15
> > samples! This might make synching mux timing and sampled data hard to
> do.
> >
>
> This delay is exactly why SAR converters are used in feedback systems.
>
> A very big advantage to sigma delta converters is that the analog
> antialiasing filters are trivial since the actual sample rate is much
> high than the "effective" sample rate.
>
> The internal digital filters may have stopband attenuation of 90dB at
> something near 1/2 the "sample rate" This of course is a major reason for
> the added delay. The better converters have better filters which have
> more delay.

I have noticed a trend recently where some of the higher-end audio converters
(especially DACs) are offering lower latency outputs, either standard, or as an
option.  Before, no one seemed to care too much or talk about it, but the
manufacturers are starting to make a bigger deal about latency/group delay.
Just thought I'd mention this.