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OpAmp

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Delta-Sigma Modulator, Feedforward or Feedback??
Jun 05^th, 2009, 8:26am

I was wondering if anybody can introduce me a document comparing the feedback and feedforward structures of delta sigma modulators?
I have read the following two books (Delta-Sigma Data Converters by Norsworthy et al and Understanding Delta-Sigma Data Converters by Schreier and Temes) regarding this topic and couldn�t find the pros and cons of the two structures. For higher order delta-sigma DACs, I usually see feedforward structure is used, but I don�t quite understand why!!!
You comments are really appreciated.

OpAmp

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Berti

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Re: Delta-Sigma Modulator, Feedforward or Feedback??
Reply #1 - Jun 23^rd, 2009, 3:37am

Hi,

I try to give you a brief overview:

Advantages of Feedforward compared to Feedback: Lower signal swing at the output of the integrators (beneficial for amplifier design), larger first loop filter coefficient (better noise suppression than feedback structure), single DAC only, and depending on the implementation the feedforward structure might recover from instability without additional circuitry.

Disadvantages: The summing node in front of the quantizer.

In general I think that the lower signal swing is the main motivation why feedforward structures are preferred nowadays.

Cheers

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Sumit Adhikari

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Re: Delta-Sigma Modulator, Feedforward or Feedback??
Reply #2 - Aug 14^th, 2009, 1:08am

Both of them has completely different usage as per as frequency domain is considered.
in feedback topology you introduce conjugate zeros in your transfer function as a result a low pass
transfer function (say in feedforward SDM) changes itself into a bandpass (feedback)
transfer function.
Lower or higher signal swing is an architectural design issue and has nothing to do
with topology. The usage of the input signal swing is decided by the choice of coefficients.
Please go through book "Data Converters" by Franco Maloberti
Regards,
Sumit

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« Last Edit: Aug 14^th, 2009, 6:20am by Sumit Adhikari »

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boa Junior Member Offline Posts: 31	Re: Delta-Sigma Modulator, Feedforward or Feedback?? Reply #3 - Aug 18^th, 2009, 7:21am Sumit, I believe you misunderstood the question: the discussion here is about 2 different architectures (feedback or feedforward) of SDM with low-pass NTF. Bandpass SDM is completely a different story..
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Berti Community Fellow Offline Posts: 356	Re: Delta-Sigma Modulator, Feedforward or Feedback?? Reply #4 - Aug 18^th, 2009, 10:47pm Yep Boa. Sumit, read the paper following paper: J.Silva et al., "Wideband low-distortion delta-sigma ADC topology", Electronics Letters. Cheers
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vivkr

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Re: Delta-Sigma Modulator, Feedforward or Feedback??
Reply #5 - Aug 20^th, 2009, 3:00am

Berti wrote on Aug 18^th, 2009, 10:47pm:

Yep Boa.

Sumit, read the paper following paper: J.Silva et al., "Wideband low-distortion delta-sigma ADC topology", Electronics Letters.

Cheers

Ah! but that is an Electronics Letters paper, a fat chance that anyone has a subscription to that one. So get it from the source directly:

http://web.engr.oregonstate.edu/~moon/research/files/el_jun07_01.pdf

Best regards,

Vivek

P.S.: There is also a lot of other useful stuff on this page. Just look around.

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