Talal Itani wrote:

> 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.


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:
> 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.

BTW, 50uH isn't really "tiny".

On Jun 22, 5:52=A0am, Paul Keinanen <keina...@sci.fi> wrote:
> On Sat, 21 Jun 2008 16:06:02 GMT, "Talal Itani" <tit...@verizon.net>
> wrote:
>
> >I was looking at the schematics for a DSP-based board, running at 100 MH=
z.
> >They have a tiny inductor with every bypass cap around the DSP. =A0Do yo=
u
> >think this is necessary? =A0This DSP has analog stuff built-in. =A0If we=
 do not
> >need analog, can the inductors be eliminated?
>
> 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.

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.



>> 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.
>
>
> 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 

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


>
> BTW, 50uH isn't really "tiny". 

Talal Itani wrote:
> 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.
> 

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.

> 
> 
> 
> 
>>
>>> 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?
>> 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
>>
> 
> 


-- 
Regards, Joerg

http://www.analogconsultants.com/

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

Talal Itani wrote:
> Ok, maybe not tiny.  I have never seen this before, so I wonder whey these 
> inductors are there.
> 

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 ;-)

> 
>> BTW, 50uH isn't really "tiny". 
> 

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:
> Talal Itani wrote:
>> 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.
>>
> Search for newsgroup postings with "Jeorg" and "ground" or "grounding" 
> in them.
> 
> You'll get a load of (AFAIK) good opinions.
> 

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


> 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.
> 

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:
> Tim Wescott wrote:
>> Talal Itani wrote:
>>> 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.
>>>
>> Search for newsgroup postings with "Jeorg" and "ground" or "grounding" 
>> in them.
>>
>> You'll get a load of (AFAIK) good opinions.
>>
> 
> Thanks for the kudos. It would have to be "Joerg" though. Sometimes I 
> wish I had an easier name.
> 
'O' before 'E' unless I'm at sea?

Dunno why I can't keep it straight.
> 
>> 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.
>>
> 
> 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.
> 
That's kinda what I thought.  Plus I see no reason to do each power line 
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:
> Talal Itani wrote:
>> Ok, maybe not tiny.  I have never seen this before, so I wonder whey 
>> these inductors are there.
>>
> 
> 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 ;-)
> 
>>
>>> BTW, 50uH isn't really "tiny". 
>>
> 
> For RF it's huge. Like a sledgehammer. Sledgehammers can cause a lot of 
> grief.
> 
WHAT?  You mean that semiconductor companies hire kids with no real 
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