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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Tony Luan <luant@xxxxxxxxxxx>
  • Date: Fri, 10 Apr 2009 22:45:56 -0700

Tony it really depends on the circumstances.  My first advice on moats 
is to avoid them.  But there are definite situations when they prove 
very valuable tools, and it is worth the trouble to deal with the side 
effects.  Companies like Charles who make low layer count boards that 
must have very high isolation between various sections would be in a bad 
way if they did not use moats.

Tony Luan wrote:
> Hi Steve,
>    I agree with you that to split GND plane is useful method for many
> situations in RF or analog circuit design. But in general, the price it
> paid in terms of routing difficulty and more noise introduced make it
> not practical for high-density high speed digital system on both package
> and PCB level.
> Thanks 
> Tony
> -----Original Message-----
> From: steve weir [mailto:weirsi@xxxxxxxxxx] 
> Sent: Thursday, April 09, 2009 6:23 PM
> To: Tony Luan
> Cc: Sol Tatlow; si-list@xxxxxxxxxxxxx
> Subject: Re: [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?
> Steve.
> 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

Steve Weir
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