Re: tiny inductor with every bypass cap?

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.

The inductors on this design are pretty bizarre and are likely not
even used on the real board.  A 50 uH inductor is rather large and
using 15 of them would stick out like a sore thumb.  I expect they
were replaced in production with 0 ohm jumpers.  In general, inductors
are not needed for power supplies.  Good ground planes do an excellent
job of minimizing high frequency noise.  Low frequency noise is
another issue however.  I used a CP Clare part once that had 0 dB
PSRR.  They didn't put that in the data sheet, they let you figure it
out on your own.  My analog power rail was derived from the digital
power rail and had 10 mV of 300 Hz noise from a control loop in the
DSP.  The 10 mV of noise coupled directly onto the phone line and
showed up as a low level, but very distinct hum or buzz.

0 dB of PSRR is unusual in a chip of any sort (I have to take my hat
off to CP Clare).  But the point is that you need to include the right
capacitance to handle noise at all frequencies.  By preventing the
noise in the first place, you don't have to worry about it coupling
into other circuits.

Rick

Rick

On Jun 21, 7:43 pm, "Talal Itani" <tit...@[EMAIL PROTECTED]
> wrote:
> 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