Reply by Alexander Sotnikov●April 7, 20112011-04-07
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
Reply by Tim Wescott●April 7, 20112011-04-07
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
Reply by steve●April 7, 20112011-04-07
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
Reply by Alexander Sotnikov●April 7, 20112011-04-07
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
Reply by Daku●April 7, 20112011-04-07
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.