[SI-LIST] Re: Split gnd planes - for/against? - Thanks!

Tony usually that is the right decision.  However, there are cases when 
a package will not fit and still provide adequate isolation between 
circuits without implementing a split.  These don't happen in digital 
designs running one signaling level. 

But suppose that you have a big digital bus switching 5A / ns that is 
0.25" away from some analog circuit that is sensitive to 10uV noise.  Do 
you think the 0.25" separation will be enough?  If you cannot change the 
placement to put the analog circuitry further away, what will you do?


Tony Luan wrote:
> Hi Sol,
> I think to split GND plane is not a good idea. 
> For any high frequency noise, GND and power plane are playing completely
> equivalent role. How can a high frequency noise tell which plane is GND?
> Which plane is power? Why not split power plane instead and keep a solid
> complete GND plane, because in package or PCB, we have tons of power
> rails and power islands already. 
> The biggest disadvantage to split GND plane I saw is that it increases
> routing difficulty when try to avoid high speed signal, clock, parallel
> bus across those gaps on GND plane. 
> Thanks 
> Tony
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Sol Tatlow
> Sent: Thursday, April 09, 2009 4:58 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Split gnd planes - for/against? - Thanks!
> One last mail to put this thread 'to bed'...
> Thanks to everyone who took the time to reply, particularly to
> those who actually gave me what I asked for, those reluctant beasts,
> the real-life examples - it's great that you're willing to so fully
> participate in such a discussion. After all, theory is great;
> coupled with real-life examples it's even better.
> Of course, the specific examples can't be applied 1:1 to any other
> design, as has been said 'x' times, but I, for one, am nevertheless
> wiser for the experiences that have been shared... I believe I'm a
> good engineer, but won't stop learning 'till the day I die.
> It was also good to see that I am not the only one suffering at the
> hands of less-than-perfect datasheets and the implementation of
> often contradictoray 'hints' :) - regardless of simulation results
> and past experiences, it's not always easy when it comes to
> persuading a customer, who is still convinced that app notes are
> written by all-knowing beings, to 'do the right thing'.
> Sol

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