Re: Re[2]: GND Plane Bounce Question

Samie

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>Text item:
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> Nirmal,
>
> Thank you for the help. I do not think that knowing the source and where
> its retrun is are crucial to answering the question as I posed it !
>
> Simply assume that I magically could inject 10 amps in 1 ns to the GND
> plane thru the via, my question is what voltage spike would you see on that
> plane ??
>
        The above would be the case if the ground plane in your design is an
        ideal GND plane and is sinking all the current. Then the Leff would be
        due to the via only and the SSN would be
                
                Vssn = Lvia * 10A/1ns

        If the above plane is NIGP ( Non-ideal Gnd plane ) then the current flow
        is going to depend on how this GND plane is connected to other reference GNDs in the design. The L due to the NIGP would depend upon
        this current flow. If you have more terminal on the GND plane your
        L due to GND plane will be lower ( becos now you have more number
        of current paths).

        Hope it makes sense. If it does not we can discuss it further
        or may be you can call me at (412-261-3200 X29).

        Regards
        
        Nirmal
        ANSOFT

> Am I missing something ? Does one really have to know the rest of teh
> topology to answer the question ?
>
> Thanks again
>
> Samie Samaan
>
>______________________________ Reply Separator _________________________________
>Subject: Re: GND Plane Bounce Question
>Author: nirmal@ansoft.com at SMTPGATE
>Date: 8/21/95 4:10 PM
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>>Text item: Text_1
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>> All IBISians (SI experts by Default !!)
>>
>> I would like to pose the following SI question, and solicit help from
>> those who actually know the answer or have a very good idea of how to
>> estimate it by scaling from previous experience:
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>> Assume one GND and one power plane in a PCB separated by say 10 mil,
>> with a dielectric constant of say 4. assume that the power plane has a
>> via opening where current is injected thru the via down to the GND
>> plane. Let's say that the current is 10 Amps. with a riserime of 1 ns.
>> Assume nominal via diameter. The drawing below shows the structure:
>>
>>
>> | Vcc
>> ---------------- | ------------------
>> | GND 10 mil, Er=4
>> -----------------|-------------------
>>
>>
>> What I would like to hear from anybody who actually has experienced
>> GND plane bounce, is:
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>> *howmuch do you expect the GND plane (not the via stem) to bounce up
>> (locally) given the above assumptions, and
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>> *howfar away from the via (radially) do you expect the bounce to drop
>> to 10% of the peak which occurs right where the via meets the plane ?
>>
>> I would apprciate a good guess (and Why: perhaps from previous
>> experience), or direction as to "who ?" you think could answer such a
>> question. I know that I might need to do a 3-D or radial transmission
>> line analysis of the system, but I'm really just looking for a quick
>> answer.
>
> The ground bounce depends on the inductance presented to the path of
> current. The correct approach is to treat the plane as multi-terminal
> conductor and solve for L matrix for all those current paths.
>
> So in the above fig you have to have another terminal or terminals
> on the GND plane which would define the current path and hence
> ground bounce. So we need the information about how the current
> flows on the gnd plane. If you have more current terminals onthe GND
> plane your effective inductance will be lower and hence GND bounce.
>
>
> Best regards
>
> Nirmal
>
>>
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>> Thanks for taking the time to read or answer this question.
>>
>> Samie Samaan
>> Intel Corp.
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>_______________________________________________________________________________
>Nirmal Jain ANSOFT CORPORATION
>(412) 261-3200 X29 (W) Four Station Sq., Suite 660
>(412) 471-9427(FAX) Pittsburgh, PA 15219
>_______________________________________________________________________________
Received on Tue Aug 22 07:54:43 1995