[SI-LIST] backplane PDN/EMC challenges
- From: qantrix <qantrix@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 23 Feb 2011 07:48:21 +0200
Hi All ,
we are challenging how to provide power/return to cards connected to
backplane
and how to deal with chassis/signal ground
So device will transfer high speed signals with Tr=0.1ns. Digital video data
rates
General curretn consumption will be 12V 40A. So like 5amps for each of
cards(8).
Device doesnt have high voltages or high energy voltage sources. general
digital video application.
There is two options - do separate polygons for each of VCC_1...8 and its
returns by using 10A common mode choke placed at power source.
Doing that supply and return currents will be under control and no return
current hot spots will be produced. and do it for all plug in cards.
Second option is doing common 12V polygone and return share with
signal(highspeed ) gnd - there will be a lot of gnd planes in 14 layer
stackup.
On plug in cards also - can be two strategies - separate polygones(control
for supply/return currents) or common - current will return with signal
return in connector)
First option is rigth from general understanding point. But in practice - do
the second variant work well? What says experience?
Another question - what is common practice for chassis gnd in rackmount
devices - does in need to be dc separated by caps from sgnd?
Many people suggest to not separate at all - that will give good emc
perfomance - all noisy gnds will be bonded with minimum inductance to metal
box(faraday cage) also esd current will have low impedance return path.
Other people say that noisy gnd planes will drive connectors area and cables
will act as efficient antenna.
I'm betweeen two opinions, by my self i think common plane for two problems
is good and simple way to go.
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