From: Nick Dietz ([email protected])
Date: Fri Feb 04 2000 - 09:47:55 PST
Hi. I've been reading this list for several months now, but I still can't
figure out what to do now. I'm a new engineer working on a new board on
which I am trying to minimize noise. Cost is not a driving issue.
The board is probably routable on two layers. It has a 3.3V-IO / 2.5V-CORE
FPGA communicating with two 5v transceiver chips. I am trying to figure out
how to do the stackup for low noise up to the 1500 Mbaud-serial /
100?MHz-parallel speeds that the board will be running at.
I'm thinking option 1:
Top (routing, and ground copper under the transceivers)
GND (full plane)
POWER (+5v under transceivers/2.5v under FPGA)
POWER (3.3v full plane)
GND (full plane)
Bottom (routing and capacitors)
This is my take on option 1: All of the signals will route with minimal
vias on the outer layers, which might be bad for emissions. The board is
only 3 inches by 4 inches, with the fastest signals moving about an inch or
so, so this should not be terribly bad.
All the signals will be directly adjacent to a solid ground plane, thus
eliminating traces passing from the 5v area to the 2.5v/3.3v area. This
should allow a good return path for all of the signals on top and bottom.
Is this a reasonable stackup?
Would it be significantly better or worse to put in a bunch more vias and
do something like this (option 2):
Top (minimal routing, ground copper under the transceivers)
POWER 3.3V
GND (full plane)
Routing
ROUTING
GND (full plane)
POWER (5v under transceivers/2.5v under FPGA)
Bottom (minimal routing)
This puts a 3.3v plane underneath all the components, even the 5v parts. Is
that a problem? Would it be better to put the split 5v/2.5v plane as the
second layer? Would it be better to switch the power layers in some other
order? (e.g. GND on layer 2 and 3.3v on layer 3, or 5v/2.5v split on layer
2? and 3.3 on layer 7)
Would the capacitance between the top layer grounded copper and the 3.3v
plane in option 2 be a bad thing since the transceivers are 5v parts?
We are not as worried about emissions as we are about having a board that
works reliably (=quietly). It will run inside a PC, so noise coming from
off the board may be a consideration.
Any more light you can shed on this issue will help a lot.
Nicholas Dietz
--- Nicholas Dietz Applied Telecom, Inc. 3060 Ogden Avenue, Suite 300 Lisle, IL 60532-1690 (630) 357-9290 x 219**** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****
This archive was generated by hypermail 2b29 : Thu Apr 20 2000 - 11:34:58 PDT