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Design >> Mixed-Signal Design >> Delta Sigma ADC questions
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Message started by kidman on Sep 11^th, 2009, 8:36pm

Title: Delta Sigma ADC questions
Post by kidman on Sep 11^th, 2009, 8:36pm

I will have my Master's thesis about Bandpass Delta Sigma ADCs. I am currently in the learning phase, so I have a few questions :

1- I noticed that most of the design and simulation time is spent on Matlab. Does that mean I will not need to use Verilog AMS?

2- Since my focus is on analog/mixed signal design. How should I implement the digital filter?

3- Since the thesis is about Bandpass Delta Sigma ADCs. Will I need to learn something more than what is needed for designing normal Delta Sigmas. Ofcourse other than learning how Bandpass works :)

4- Does the integrator actually consist of an Opamp or is an OTA preferred?

5- What is the block that I will probably spend most of my time designing?

Title: Re: Delta Sigma ADC questions
Post by Berti on Sep 16^th, 2009, 4:15am

Kidman, I think that most of your question cannot be conclusively answered. Often there are both advantages and disadvantages whether you e.g. use an OpAmp or OTAs. I would recommend you to sit together with your advisor.

Regards

Title: Re: Delta Sigma ADC questions
Post by kidman on Oct 14^th, 2009, 6:21pm

Ok I have simple fundamental question. Why can't we do noise shaping unless we are oversampling?

Title: Re: Delta Sigma ADC questions
Post by ywguo on Oct 14^th, 2009, 7:48pm

kidman wrote on Oct 14^th, 2009, 6:21pm:

Ok I have simple fundamental question. Why can't we do noise shaping unless we are oversampling?

Hi Kidman,

Let's think it over. Where do you want to put the shaped noise, which is very large, in the frequency spectrum if it is not oversampled.

Yawei

Title: Re: Delta Sigma ADC questions
Post by kidman on Oct 15^th, 2009, 4:22am

I mean why not do noise shaping without oversampling. Why don't we use the same concept of noise shaping in Nyquist rate converters

Title: Re: Delta Sigma ADC questions
Post by thechopper on Oct 16^th, 2009, 10:40am

Kidman,

As YAwei suggested, if you consider Nyquist sampling rate, due to the bandwidth of the noise you are gonna get of lot of noise power into your baseband, unless you oversample. That is why.

Best
Tosei

Title: Re: Delta Sigma ADC questions
Post by kidman on Oct 16^th, 2009, 1:46pm

I agree with that, but still, why not do noise shaping in nyquist rate converters to decrease the noise further?

Title: Re: Delta Sigma ADC questions
Post by Berti on Oct 19^th, 2009, 3:57am

Noise shaping requires oversampling: Noise shaping and Nyquist rate conversion therefore don't work together!!!!

Nevertheless, I think it would theoretically be possible to combine ADC topologies which are typically used at Nyquist such as Pipeline, Folding etc. with noise shaping (for example replace flash of a Delta-Sigma ADC with Pipeline or Folding - or a 2-0 cascaded Delta-Sigma etc.). Even though most of this solution probably are not very appealing.

Cheers

Title: Re: Delta Sigma ADC questions
Post by pirate on Oct 20^th, 2009, 9:35am

Like Berti is saying noise shaping in nature requires oversampling.

To my knowledge noise shaping can only occur in the delta sigma structure where the quantization error is processed by the loop filter which in the case on the delta modulation is an high pass filter (integrator; delta modulation).

Therefore if the sampling frequency is much higher than than the signal frequency, noise is being greatly reduced or shaped in the band of interest (ie: signal band).

Ideally you can operate a delta sigma slightly above the Nyquist rate provided an ideal brick wall high pass loop filter which is pretty much impossible to built.

I would recommend the first chapter Richard Shreier book and the original paper on the delta modulation

[http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471465852.html]

[H. Inose, Y. Yasuda, and J. Murakami, "A telemetering system by code modulation-Δ-Σ modulation." <i>IRE Transactions on Space Electronics and Telemetry, vol. 8, pp. 204-209, 1962]

Title: Re: Delta Sigma ADC questions
Post by kidman on Oct 20^th, 2009, 11:24am

Thanks