Hello,

I was looking at the schematics for a DSP-based board, running at 100 MHz.
They have a tiny inductor with every bypass cap around the DSP. Do you
think this is necessary? This DSP has analog stuff built-in. If we do not
need analog, can the inductors be eliminated?

Thanks,
T.I.

Talal Itani wrote:
Seems overkill.
ADI reference designs don't use anything like that, although they follow
PSU chips with an inductor.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

Talal Itani wrote:
Great, which DSP ?

Depends, which DSP ??

This DSP has analog stuff built-in. If we do not
It must be a secret, what does the manufacture of the DSP say ??


donald

Talal Itani wrote:
in them.

You'll get a load of (AFAIK) good opinions.

Inductors in series with the caps would tend to isolate the power supply
from noise in the DSP, but it would also create a bunch of odd
resonances. It's not how I'd want to isolate a power supply from a chip.

--

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

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

On Sat, 21 Jun 2008 16:06:02 GMT, "Talal Itani" <titani@verizon.net>
wrote:

Inductors don't generally help digital chips, and may actually reduce
timing margins. We do use ferrite bead+capacitor filters on the supply
rails of some fast opamps and adc's, to keep switcher noise and
other-channel crosstalk from sneaking in.

The best way to power big digital chips is with solid power planes,
reasonably bypassed. That will present lower rail impedances than you
could get by isolating the bypass caps on a per-pin basis.

John

On Jun 21, 12:06 pm, "Talal Itani" <tit...@verizon.net> wrote:
Is it possible the inductors represent the inductance of the layout,
rather than actual parts on the BOM?

On Sat, 21 Jun 2008 16:06:02 GMT, "Talal Itani" <titani@verizon.net>
wrote:


Are you sure that these are ordinary inductors or just a wire through
a ferrite bead?

While the ferrite will increase the inductance, a suitable ferrite
material is also quite lossy at higher frequencies, reducing the risk
for unwanted resonances with the capacitors.

Paul

The DSP is a TI F2808. The schematics I was referring to are here
http://www.ti.com/litv/zip/sprr098. It is a zip file. Once you unzip the
file, 2 pdf files appear. The larger file has the schematics I am referring
to. The inductors are at the top-left corner of the screen.

I cannot tell. I do not have BOM. The schematics are here, a zip file
that has 2 PDF files. The large PDF file has the schematics. The inductors
are at the top-left corner of the page. Thanks.

On Jun 21, 4:14 pm, "Talal Itani" <tit...@verizon.net> wrote:
That's interesting, the resonance frequency for 100nF with 50uH is
about 21KHz. I wonder what the resonance-decoupling-multi-value caps
crowd has to say about this design?

Talal Itani wrote:


This mediocre design is obviously made by a superstitious and
unexperienced person. There are several things in the schematics that
should be done differently. No wonder that at some time ago the designer
had burned with the EMC, and after that he sticks the inductors
everywhere. The value of 50uH is ridiculous. Never mind those inductors;
with the sensible layout the F28xx doesn't need them.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On Jun 21, 12:06 pm, "Talal Itani" <tit...@verizon.net> wrote:
BTW, 50uH isn't really "tiny".

On Jun 22, 5:52*am, Paul Keinanen <keina...@sci.fi> wrote:
Sadly, you can't rely on this. I've had to put little resistors in
series with ferrite bead to kill a resonance - admittedly at a few
hundred kHz, where the bead doesn't look that lossy.

--
Bill Sloman, Nijmegen

What if we are not sensible about the layout? Meaning, I layout the board
myself.

Ok, maybe not tiny. I have never seen this before, so I wonder whey these
inductors are there.

Talal Itani wrote:
To be honest 50uH and 0.1uF is a recipe for disaster. At the most a
ferrite SMT-bead should be used but usually I don't even do that. A nice
full ground plane and a nice full VCC plane is usually best. Problem
with DSP like this is that you need an additional lower voltage supply
so now you are up to three supply planes, meaning you won't get away
with less than a 6-layer board.

If it's super critical you could have the analog supplies come from a
separate regulator but often converters on a chip with fast digital
processing going on are quite disappointing. A bond wire affords only so
much in RF conductivity.

Hint: Carefully read up on power supply sequencing. Best case wriobng
sequencing leads to a locked up DSP, worst case to a dead DSP.

Oh, and please don't top post.


--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Talal Itani wrote:
Possibly a very young guy did the design. There are people who take a
sledgehammer to hang a picture. Sometimes the sledgehammer then makes a
hole in the wall ;-)

For RF it's huge. Like a sledgehammer. Sledgehammers can cause a lot of
grief.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Tim Wescott wrote:
Thanks for the kudos. It would have to be "Joerg" though. Sometimes I
wish I had an easier name.


It will become really interesting when the DSP exhibits a somewhat
burst-like load behavior. On the scope it'll look like Dolphins
frolicking in the ocean.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Joerg wrote:

Dunno why I can't keep it straight.
individually, and some good reasons not to (Different versions of VDD at
different points in the circuit, oh boy!).

--

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

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

Joerg wrote:
experience fresh out of college to be applications engineers?

Now THAT would imply that they look at their applications engineers as a
marketing expense, not a profit center.

--

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

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