[SI-LIST] Re: Split gnd planes - for/against?
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Duane Mattheisen <duane.mattheisen@xxxxxxx>
- Date: Tue, 07 Apr 2009 16:56:51 -0400
Duane,
Your example reminded me of one of mine. 9 years ago I was designing
the package for a 10 Gbps chip, in an Organic wire bond package, that
included single-ended forwarded 10 GHz clocks. Parasitic ground
coupling through the IC substrate excited a fundamental resonance on the
package ground plane, since the parasitic impedance was lower than the
wire bond impedance in the 10 GHz region. The solution was a slot in
the ground plane underneath the bond wires, to block the parasitic mode,
and force the signals to pass across the slot on co-planar bond wires.
Scott
Duane Mattheisen wrote:
> I had a combiner limiter that had about 100dB of gain, it was used in a
> satilite reciever. There were three aplifier stages to bring up the gain to
> 100dB. Oscilations would happen because the last output stage's current
> would run through the ground plane and pass by the input of the first stage.
> The oscilations were stoped by making a trench in the ground plane at the
> middle stage of the amplifiers and moving the ground wire to the bottom of
> the trench.
>
>
> Regards, Duane Mattheisen
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Lee Ritchey
> Sent: 2009 Apr 07 11:28 AM
> To: Steve Weir
> Cc: Charles Grasso; Sol Tatlow; si-list
> Subject: [SI-LIST] Re: Split gnd planes - for/against?
>
> Steve,
>
> You have described a hypothetical case here. I'm looking for a real one.
>
> You didn't reply with "you will have to get a case yourself" as some do.
> Refusing to supply an example to support claims is tantamount to making
> things up.
>
> If a claimer can't show a case where a rule works, the claim should not be
> made.
>
> Lee
>
>
>
>> [Original Message]
>> From: steve weir <weirsi@xxxxxxxxxx>
>> To: <leeritchey@xxxxxxxxxxxxx>
>> Cc: Charles Grasso <Charles.Grasso@xxxxxxxxxxxx>; Sol Tatlow
>>
> <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx>
>
>> Date: 4/7/2009 11:13:14 AM
>> Subject: Re: [SI-LIST] Re: Split gnd planes - for/against?
>>
>> Lee and this will be true in the majority of cases, and almost without
>> exception where a board is digital only. For an example of where a
>> split works, and it must be done properly or all bets are off:
>>
>> Circuit region 1, noise sensitivity is in the uV's. Let's assume the
>> power / ground separation is 4 mils. The spreading inductance is
>> roughly 128pH / square. Now assume that there is an adjacent circuit
>> region that has a 64 bit memory bus on it. Let's assume very pedestrian
>> 200ps rise times on 18mA swing. That bus is switching just over
>> 5E9A/s. If circuit region 1 has a noise limit of 10uV across its length
>> and a square aspect ratio, then we need over 90dB isolation. We aren't
>> going to get there with placement and bypass caps alone. But we can get
>> there by including well designed moating
>>
>> You are absolutely correct that many people put moats in they don't need
>> and worse do so in a way that creates other more serious problems.
>> Moats are sort of like ferrite beads: they are tools that have
>> particular value is specific circumstances. They always come with a
>> price. Their must be justified as needed and appropriate, and once used
>> engineered correctly.
>>
>> Best Regards,
>>
>>
>> Steve.
>> Lee Ritchey wrote:
>>
>>> I've taught my high speed class to more than 7000 engineers and
>>>
> designers
>
>>> over the years. In each class, I ask for examples where splitting a
>>>
> ground
>
>>> plane actually made a circuit work better with the promise to add the
>>> example to my class.
>>>
>>> To date, there have been no examples provided. There has been a bit of
>>> hand waving on the topic but no clear examples that can be defended.
>>>
> The
>
>>> usual reason is "we've done it this way for years and it has worked."
>>>
> To
>
>>> me, that sounds an awful lot like the man who jumped off the 20 story
>>> building and reported as he passed the 10th floor, "so far, so good".
>>> Splitting the ground plane just hasn't shown up as a problem, not that
>>>
> is
>
>>> actually fixed anything.
>>>
>>> I have fixed EMI problems several times by removing splits in ground
>>> planes. Some of see this as easy money!
>>>
>>> I'll make the same offer to this group. Show me an example where
>>>
> splitting
>
>>> ground planes helps and I'll make it a part of my course.
>>>
>>> Lee Ritchey
>>>
>>>
>>>
>>>
>>>> [Original Message]
>>>> From: Grasso, Charles <Charles.Grasso@xxxxxxxxxxxx>
>>>> To: steve weir <weirsi@xxxxxxxxxx>; Sol Tatlow
>>>>
>>>>
>>> <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
>>>
>>>
>>>> Cc: si-list <si-list@xxxxxxxxxxxxx>
>>>> Date: 4/7/2009 9:57:52 AM
>>>> Subject: [SI-LIST] Re: Split gnd planes - for/against?
>>>>
>>>> We use split planes all the time.
>>>> When you have circuits of *vastl8 different noise floors co-existing
>>>> on one board - it's the only way to go.
>>>>
>>>> Chas
>>>>
>>>> -----Original Message-----
>>>> From: si-list-bounce@xxxxxxxxxxxxx
>>>>
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
>
>>>> On Behalf Of steve weir
>>>> Sent: Tuesday, April 07, 2009 10:11 AM
>>>> To: Sol Tatlow
>>>> Cc: si-list
>>>> Subject: [SI-LIST] Re: Split gnd planes - for/against?
>>>>
>>>> Sol, unfortunately there is not a single answer. In most cases
>>>>
> moating
>
>>>> is a bad idea, particularly if one does not understand the caveats and
>>>> how to deal with them. It's not just the moats: It's the placement,
>>>> clearances, stitching, and routing that all need to be considered.
>>>>
>>>> Steve
>>>>
>>>> Sol Tatlow wrote:
>>>>
>>>>
>>>>> I know this subject has been raised before, countless times in one
>>>>> guise or another. I have also googled plenty. I'm not looking for
>>>>> theoretical opinions, either, about whether or not, or when, they
>>>>> should be used (specifically not, "it depends", unless you've got
>>>>> REAL-LIFE examples, for and against!!!).
>>>>>
>>>>> This subject raised its head for me in this case due to using
>>>>> 2 A/Ds as well as 2 D/As, both from Analog Devices, where one
>>>>> specifies a split plane, the other specifies no split. Now, I am
>>>>> all too wary of relying simply on evaluation boards, where, in
>>>>> general, one layout is done, and if it works, that's how everyone
>>>>> should do it (_without_ comparing 2 different approaches).
>>>>>
>>>>> I personally have 3 concrete cases where split gnds had no positive
>>>>> effect on SI, but significantly worsened EMC results (despite
>>>>> sticking to all the usual guidelines, like no tracks over the
>>>>> splits, etc.), but I have no concrete case FOR split ground planes.
>>>>>
>>>>> So, what I'm interested in is: does anyone have CONCRETE examples
>>>>> which they would like to share for/against split planes? The kind
>>>>> of thing I mean would be like in one of the cases I had, where I
>>>>> wanted to go against the suggested approach of using a split gnd,
>>>>> and persuaded my customer to pay for 2 variants of the same board
>>>>> on the same manufacturing panel, one with split ground, one with
>>>>> solid ground. Both variants were assembled and tested, with regards
>>>>> to both SI as well as EMC: both were functionally satisfactory; at
>>>>> EMC testing, however, the split-plane bombed out big time, while
>>>>> the non-split sailed through. I like to think that it wasn't due
>>>>> to any screw-ups on my side, that the split ground failed - I am
>>>>> not a newbie to PCB layouts, and, while for sure no professional
>>>>> expert on all areas of SI, I believe I avoided the typical blunders
>>>>> often present in split ground layouts.
>>>>>
>>>>> Anyway, my customer was more than happy, but not everyone has the
>>>>> money/time/desire to do as I suggested. So, any 'war stories' to
>>>>> support one or the other approach would be appreciated to help
>>>>> expand my knowledge and understanding of this subject - obviously,
>>>>> we all respect confidentiality, so I'm not looking for circuits,
>>>>> layouts and so on, but I figure many of you must have stories that
>>>>> can be related regarding this subject. Or perhaps some good links
>>>>> to non-confidential 'real-life' examples/studies?
>>>>>
>>>>> Regards,
>>>>> Sol
>>>>>
>>>>>
>>>>>
>>>>>
>>>> --
>>>> Steve Weir
>>>> Teraspeed Consulting Group LLC
>>>> 121 North River Drive
>>>> Narragansett, RI 02882
>>>>
>>>> California office
>>>> (866) 675-4630 Business
>>>> (707) 780-1951 Fax
>>>>
>>>> Main office
>>>> (401) 284-1827 Business
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>>>>
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>>>>
>>>> http://www.teraspeed.com
>>>> This e-mail contains proprietary and confidential intellectual property
>>>> of Teraspeed Consulting Group LLC
>>>>
>>>>
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>>>
>>>
>>>
>> --
>> Steve Weir
>> Teraspeed Consulting Group LLC
>> 121 North River Drive
>> Narragansett, RI 02882
>>
>> California office
>> (866) 675-4630 Business
>> (707) 780-1951 Fax
>>
>> Main office
>> (401) 284-1827 Business
>> (401) 284-1840 Fax
>>
>> Oregon office
>> (503) 430-1065 Business
>> (503) 430-1285 Fax
>>
>> http://www.teraspeed.com
>> This e-mail contains proprietary and confidential intellectual property
>>
> of Teraspeed Consulting Group LLC
>
> ----------------------------------------------------------------------------
> --------------------------
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>> Teraspeed(R) is the registered service mark of Teraspeed Consulting Group
>>
> LLC
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
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