tiny inductor with every bypass cap?

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

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

Search for newsgroup postings with "Jeorg" and "ground" or "grounding" 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.

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

Is it possible the inductors represent the inductance of the layout, rather than actual parts on the BOM?

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

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.

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?

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

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

z.

do not

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.

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.

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.

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.

'O' before 'E' unless I'm at sea?

Dunno why I can't keep it straight.

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!).

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.