MK wrote:

> I designed a board with an 'F2808 about 2 years ago.  Based on TI app notes I
> put 100nH in series with the two ADC power pins only and 2uF to ground on 
> each (2 * 0603 caps - different values).
> 
> Was it necessary - I don't know - never tried it without.
> 
> Did the ADC work - yes - I was impressed at how well it worked.

The internal ADC of TI 28xx is crap. It has tremendously inacurate 
internal reference and high zero offset. With 12 bits available, the 
true accuracy is about 8 bits unless you have the ADC calibrated.

> The board was 4 layers with careful design of gorund planes/fills around the 
> processor and decoupling caps on both sides.

There is no need for 4 layer board with 28xx. The layout can be done on 
two layers just fine. Been there, done that.


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

Talal Itani wrote:

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

If you are not sensible about the layout, then God help you.


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




> 
>>>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 
> 
> 
> 

rickman wrote:

> If you are going to layout the board yourself, then you need to learn
> good board design techniques.  Even if you let someone else layout the
> board, you should be the one telling them what to do, not the other
> way around.  To do otherwise can result in a board that does not work
> because of power decoupling and SI issues.

It is not a big deal to lay out the board for TMS28xx. Two layers work 
fine. With 28xx, the very important thing is the power sequensing on 
startup and shutdown.

> The inductors on this design are pretty bizarre and are likely not
> even used on the real board.

Indeed. Especially as the power pins of the chip are supposed to be 
connected directly.


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

>
>> If you are going to layout the board yourself, then you need to learn
>> good board design techniques.  Even if you let someone else layout the
>> board, you should be the one telling them what to do, not the other
>> way around.  To do otherwise can result in a board that does not work
>> because of power decoupling and SI issues.
>
> It is not a big deal to lay out the board for TMS28xx. Two layers work 
> fine. With 28xx, the very important thing is the power sequensing on 
> startup and shutdown.
>
>> The inductors on this design are pretty bizarre and are likely not
>> even used on the real board.
>
> Indeed. Especially as the power pins of the chip are supposed to be 
> connected directly.


Yes.  Bizarre is the right word.  Two layers work fine for TI 20xx?  No 
ground plane, no voltage supply lanes?

What about using one of TI's power ICs, with 3.3V  1.8V and Reset Generator?

T.I. 

"Vladimir Vassilevsky" <a...@hotmail.com> wrote in message 
news:hzA7k.5249$L_.3...@flpi150.ffdc.sbc.com...
>
>
> Talal Itani wrote:
>
>> What if we are not sensible about the layout?  Meaning, I layout the 
>> board myself.
>
> If you are not sensible about the layout, then God help you.
>
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultant
> http://www.abvolt.com
>

I feel, at times God helps me, and at times he works against me.  Oh Well, I 
have to keep trying.  In your previous post you said that 2 layers is good 
enough for TI 28xx.  I did layout some low-speed microcontroller 2 layer 
boards in the past.  Do you think a 100 MHz DSP board requires different 
skills.  The board does not have much on it.  The DSP, and a few other plain 
chips.

Thanks,
T.I. 

On Jun 23, 6:46=A0am, Joerg <notthisjoerg...@removethispacbell.net>
wrote:
> Paul Keinanen wrote:
> > On Sat, 21 Jun 2008 15:25:38 -0700 (PDT), bill.slo...@ieee.org wrote:
>
> >> On Jun 22, 5:52 am, Paul Keinanen <keina...@sci.fi> 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.
> >> 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.
>
> > It would be quite hard to find material that would be lossy at such
> > low frequencies. For instance materials listed at
> >http://www.fair-rite.com/newfair/materials.htmstart at 1 MHz
> > (material 31), while the common material 73 have significant losses
> > only above 10 MHz.
>
> I am lossy at low frequencies. Met my wife at an ultrasound company and
> she was quited stunned that my body would absorb even 3.5MHz to the
> point where she couldn't get a decent ultrasound image. Never seen it
> this bad before. She married me anyway.

One of my colleagues in the ultrasound group at EMI was a rower with
lots of muscle around the trunk - made him a lousy test subject for
ultrasound imaging. I played field hockey, which mainly builds up the
running muscles, and was much more transparent.

--
Bill Sloman, Nijmegen

Tim Wescott wrote:
> 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!).
> 

they are also probably ferrite beads, rather than inductors per se. but 
I have corrected a few designs where the "engineers" really did use 
inductors. nasty little bobbin core things. yuk!

I've seen quite a few app notes with FBs liberally sprinkled everywhere. 
I presume this is because its easier than thinking. that being said, I 
have a design thats soon to undergo EMC testing where I have exactly 
followed the manufacturers recommendations for the FPGA & HY ships, but 
I plan on muntzing most of the FBs during a day at the EMC lab. odds on 
I can leave ALL of them off...

Cheers
Terry

Joerg wrote:
> John Larkin wrote:
> 
>> On Sun, 22 Jun 2008 13:03:09 +0000, Guy Macon
>> <http://www.guymacon.com/>; wrote:
>>
>>>
>>>
>>> John Larkin 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.
>>>
>>> Unless my design is cost sensitive, I generally put ferrite beads
>>> on the wires coming from the power supply.  I have never had this 
>>> actually change how well anything works, but I can see a bit less 
>>> noise with a scope, and doing it that way is fairly cheap.  I have
>>> seen all sorts of schemes for power/ground, but I have never seen any 
>>> thing that beats a solid ground plane and good physical placement
>>> of sensitive / noise-producing sections.
>>
>>
>> Any time you add inductance to a supply rail, the consequences should
>> be analyzed. It could...
>>
>> Series resonate at some switching supply frequency and *increase*
>> downstream ripple
>>
>> Add DC drop. With chip core voltages below 1 volt, and timing margins
>> critical, that could get interesting
>>
>> If a chip can grossly shift its average current needs (like a uP, or
>> some analog driver) the added L can make the supply dip.
>>
> 
> Or spike. Old rule with inductors: They want to maintain their current 
> no matter what. If they can't then they do what taxing authorities do 
> when they run out of dough: Raise the voltage ...
> 
> 
>> The inductor could fry!
>>
>>
>> I've seen all of these.
>>
> 
> Seen some grief in that domain as well. Including freaking expensive 
> FPGA that have become doorstops.
> 

One design I looked at used a 600MHz agilent parallel/serial - 
serail/parallel chipset. there was a 1uH inductor between the 0V plane 
of the receiver/equaliser chip & the agilent serial/parallel chip 0V plane.

oddly enough the serial link seemed a bit flaky. that didnt stop the 
company from selling hundreds of millions of dollars worth of product, 
but it did meant they could daisy chain 2 units, rather than the dozens 
they planned on.

and there was no way I could convince their head of engineering this was 
a bad thing. OTOH they paid my bill, and I did fix a whole host of other 
problems.

Cheers
Terry

"Talal Itani" <t...@verizon.net> wrote in message
news:QJB7k.965$9J.528@trnddc06...
>
> >
> > It is not a big deal to lay out the board for TMS28xx. Two layers work
> > fine. With 28xx, the very important thing is the power sequensing on
> > startup and shutdown.
> >
> >> The inductors on this design are pretty bizarre and are likely not
> >> even used on the real board.
> >
> > Indeed. Especially as the power pins of the chip are supposed to be
> > connected directly.
>
> Yes.  Bizarre is the right word.  Two layers work fine for TI 20xx?  No
> ground plane, no voltage supply lanes?

It depends.
Really nothing special is required for TMS28xx for the EMI and the signal
integrity purposes. However if the board is supposed to sustain the BCI
test, then  you will have to use four layers or more.

>
> What about using one of TI's power ICs, with 3.3V  1.8V and Reset
Generator?

Don't know about the dual regulators, never used those. Check the sequensing
specs very carefully; it could be the false states at the outputs at
startup/powerdown.

Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com

rickman wrote:
>
>On Jun 22, 9:03 am, Guy Macon <http://www.guymacon.com/>; wrote:
>
>> John Larkin 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.
>>
>> Unless my design is cost sensitive, I generally put ferrite beads
>> on the wires coming from the power supply.  I have never had this
>> actually change how well anything works, but I can see a bit less
>> noise with a scope, and doing it that way is fairly cheap.  I have
>> seen all sorts of schemes for power/ground, but I have never seen
>> any thing that beats a solid ground plane and good physical placement
>> of sensitive / noise-producing sections.
>
>I am curious about this.  I can see the theory behind adding
>inductance to individual branches of the supply to prevent noise from
>one section from reaching other section of the board.  But to add
>inductance to the common supply line seems like it would only make
>things worse.  The noise you saw a reduction in, was it from the
>supply rather than from the board?

Yes.  Switching power supply hash.  


-- 
Guy Macon
<http://www.GuyMacon.com/>;