[SI-LIST] Re: Split Plane Routing
- From: Bob Patel <whizplayer@xxxxxxxxx>
- To: doug.hopperstad@xxxxxxxxxx, psharkey@xxxxxxxxxxx,"Si-List@Freelists. Org" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 1 Feb 2002 11:49:44 -0800 (PST)
Hi! I agree with the responses that it is best not to
cut up a plane. I have also proved to myself
practically by eliminating cut up planes on a respin
board and using continuous ground and there was a
slight difference in the performance i.e. it improved.
I deduce this by comparing EYE mode between two
brds,also the two brd were identical in the area where
I took EYE measurements except the ground planes.
In your case where the FPGA interfaces to the ADC you
want to isolate the two grounds i.e. DGND under FPGA
and AGND under ADC. How are your bypass capacitors
connected on the ADC? If it is to DGND then you
created a big ground loop. Now say you connect them to
AGND but the interface from FPGA to ADC is digital but
whenever the signals in the ADC(connected to FPGA)
switch they use transient power from the bypass but
which ones?
Now the mfgr of ADC might specify VDD(digital power)
and AVDD(analog power) and then it becomes bit
simpler.
But you have to consider any potential difference
between the two grounds depending upon how you tie
them: Star or Mesh.
The point is if you know where the return currents for
your signals are then life becomes easier otherwise
you keep guessing.
Just my 0.02
Bob
--- Doug Hopperstad <doug.hopperstad@xxxxxxxxxx>
wrote:
>
> I would be more concerned with splitting the ground
> plane than what =
> layers to use. Splitting a ground plane can cause
> return current loop =
> problems. It is best not to cut up the ground planes
> if all possible.
>
> Doug Hopperstad
> Qlogic
> 6321 Bury Drive
> Eden Prairie, MN
> Phone: (952) 932-4007
> Fax: (952) 932-4037
> doug.hopperstad@xxxxxxxxxx
>
>
>
>
> -----Original Message-----
> From: Pat Sharkey [mailto:psharkey@xxxxxxxxxxx]
> Sent: Friday, February 01, 2002 8:10 AM
> To: Si-List@Freelists. Org
> Subject: [SI-LIST] Split Plane Routing=20
>
>
>
> Hello all,
>
> Given the following stackup with planes split in
> different areas -=20
>
> 1 - S1
> 2 - GND (SPLIT 1)
> 3 - S2
> 4 - GND(SPLIT 2)
> 5 - POWER(SPLIT 3)
> 6 - S3
> 7 - GND (SPLIT 4)
> 8 - S4.
>
> For a signal which originates on layer 8, should the
> signal only be
> routed on layers 6 & 8? Or, can layers 1 and 3 also
> be used to cross
> a plane split on layer 7, assuming layers 2 and 4
> are not split in the=20
> same place (z axis)?
>
> Thanks in advance,
> Pat
>
>
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