Low pass filter for sigma delta ADC

Could some DSP guru please help ? For a sig-delta ADC, how is the cut-
off frequency
for the low-pass filter determined, given that
there is also a decimation filter just after that.
Any hints, suggestions would be of immense
help. Thanks in advance.

On 07.04.2011 7:56, Daku wrote:
> Could some DSP guru please help ? For a sig-delta ADC, how is the cut-
> off frequency
> for the low-pass filter determined, given that
> there is also a decimation filter just after that.
> Any hints, suggestions would be of immense
> help. Thanks in advance.
I'm not even near a guru, but i can tell you that the cutoff frequency 
of the composite response should be lower than the point where the Noise 
Transfer Function starts rising. Usually no one performs filtering in 
SDM in one step. Most often the data stream that the modulator produces 
is filtered by combination of sinc (running average) or CIC filter, 
because they are easier to implement in hardware, and one or several FIR 
stages. The way you choose their cutoff frequencies and decimation 
ratios depends on  both the required in-band SNR (because after 
decimation all the SDM noise that is not sufficiently attenuated gets 
aliased into the 1st Nyquist zone) and hardware constraints (speed, 
area, power consumption etc).

You can read more on this topic, for example, in
http://www.amazon.com/Understanding-Delta-Sigma-Converters-Richard-Schreier/dp/0471465852
and
http://www.amazon.com/Delta-Sigma-Data-Converters-Theory-Simulation/dp/0780310454


If you need a more specific answer then try to reword your question so 
that it would be a bit more informative.

-- 

Alexander

On Apr 6, 11:56=A0pm, Daku <dakup...@gmail.com> wrote:
> Could some DSP guru please help ? For a sig-delta ADC, how is the cut-
> off frequency
> for the low-pass filter determined, given that
> there is also a decimation filter just after that.
> Any hints, suggestions would be of immense
> help. Thanks in advance.

First decide how much of the high band aliased signal you can
tolerate.

Say it's 1% as an example.

Then determine from the datasheet where the stopband (from the
decimation filter) extends out to, say 20Mhz.

Design your antialiasing filter so that at 20Mhz it has at max gain of
1/100.

Usually a first or second order pole filter is all that is needed

On 04/07/2011 12:00 AM, Alexander Sotnikov wrote:
> On 07.04.2011 7:56, Daku wrote:
>> Could some DSP guru please help ? For a sig-delta ADC, how is the cut-
>> off frequency
>> for the low-pass filter determined, given that
>> there is also a decimation filter just after that.
>> Any hints, suggestions would be of immense
>> help. Thanks in advance.

(good specific advice snipped)

> If you need a more specific answer then try to reword your question so
> that it would be a bit more informative.

Like, tell us from a systems perspective what you're trying to _do_. 
Are you building a sigma-delta converter from scratch?  Are you using a 
commercial one?  Is there some hybrid S-D solution out there that lets 
you buy the modulator but leaves you hanging as far as building the filter?

I didn't address the original post because I'm not much of a S-D guru 
either (I've got a number of 1st-order S-D converters fielded, and they 
do great in narrow circumstances*, but that's baby talk compared to real 
S-D technology).

What I do know is that there is a great deal of interaction between a 
S-D converter's front end and the filter -- so great, that I suspect 
that the real answer to your question is "analyze the front end behavior 
and choose a filter that makes your converter meet spec".  If there are 
rules of thumb, I rather suspect that they only apply to specific 
front-end architectures, not to any old S-D modulator.

* 
http://www.eetimes.com/design/embedded/4006431/Sigma-delta-techniques-extend-DAC-resolution,
http://www.wescottdesign.com/articles/sigmadelta.html

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

On 4/7/2011 6:51 PM, steve wrote:
> On Apr 6, 11:56 pm, Daku<dakup...@gmail.com>  wrote:
>> Could some DSP guru please help ? For a sig-delta ADC, how is the cut-
>> off frequency
>> for the low-pass filter determined, given that
>> there is also a decimation filter just after that.
>> Any hints, suggestions would be of immense
>> help. Thanks in advance.
>
> First decide how much of the high band aliased signal you can
> tolerate.
>
> Say it's 1% as an example.
>
> Then determine from the datasheet where the stopband (from the
> decimation filter) extends out to, say 20Mhz.
>
> Design your antialiasing filter so that at 20Mhz it has at max gain of
> 1/100.
>
> Usually a first or second order pole filter is all that is needed


Oops, that's what happens if someone is taken up with one particular 
application. Being involved primarily in design of sigma-delta 
modulators for DACs i completely forgot about such thing as the 
antialiasing filter :)
If that is the lowpass filter mentioned in the original post, then this 
article may also be helpful
http://www.analog.com/library/analogDialogue/Anniversary/15.html

--
Alexander