# [SI-LIST] Re: Seperate Sparameter for gnd & supply plane !!

• From: Young <long.0.yang@xxxxxxxxx>
• To: Rajan Hansa <all.si.list@xxxxxxxxx>
• Date: Thu, 2 Jun 2011 16:08:56 +0800

```What we care about and what it makes sense is the voltage difference
between vcc and vss at chip side. Do not leave vcc or vss alone.

Generally when extracting S-parameter we need define local positive
terminal and negative terminal(as reference 0, mostly is ground).
Keeping this concept in mind, the S-parameter captures the loop
parasitics, including all the effect between the local positive
terminal and the negative terminal. In the other words, S-parameter
includes the parasitics between vcc and vss from the local terminals.
Or you can try to swap the positive terminal and negative terminal,
you will have the same S-parameter, because the loop is the same.

Regards
Long

On Thu, Jun 2, 2011 at 01:04, Larry Smith <LSMITH@xxxxxxxxxx> wrote:
> Rajan - There are some very basic principles that should be considered here.
>  To the little circuits on the die, the only PDN voltage that is important is
> the difference between Vcc and Vss locally at the circuit terminals.  The
> circuits respond to this voltage and they do not care about the voltage on
> the PCB ground plane or the voltage at the center of the earth or any other
> place.
>
> An S parameter model of the PDN is fully capable of giving Vcc-Vss at the
> circuit terminals in a properly set up simulation.  The simulation may show
> you local Vcc-Vss (differential) at several points in the system (i.e. bump
> joints, wire bond pads, package-via tops and bottoms, package balls, pcb
> vias, pcb power planes, etc.).  At each of these points in the system, the
> only thing that is important is the difference between Vcc and Vss voltage.
>  Consider this the "TEM" for the PDN if you like.  The voltage along the
> ground path is not defined (voltages are not unique and depends greatly upon
> the path of integration).  The structures are usually large enough that
> voltage along a ground path distance should be considered voltage across time
> which is not well defined.  S parameters cut through all that and give you
> the impedance of some port when a different port is excited, always with
> respect to a local reference node.
>
> talk about "ground noise."  But the voltage on a node (particularly ground)
> is not defined (what would you measure it with respect to?).  Voltage is
> always the difference in potential between two nodes (i.e. Vcc-Vss) and is
> clearly defined when you can identify a parallel plate capacitor to integrate
> the E field through a distance (volts/meter x meters).  This is valid when
> there are no time varying magnetic fields in the picture.  All voltage bets
> are off when you get into typical packaging structures where there certainly
> are time varying magnetic fields and the integral of dB/dt through a loop
> area is important. Once again, the local Vcc-Vss voltage is still valid all
> along the PDN path (TEM mode) but voltage along the path (and therefore
> ground noise) is not defined.  An S parameter model simulation that has a
> port for Vcc wrt it's local Vss will tell you everything you need to know
>  wer quality for the local circuit..
>
> Now, what about the signals that come away from the die circuit and follow
> some path down to a PCB trace?  As long as the signals are referenced to
> ground along the entire path, the S parameter representation of that path is
> good.  But if the signal finds itself referenced to Vcc or some other node
> along the way (return current is on something other than continuous ground),
> then you have to be real careful about mode conversion when the signal
> return-current is forced to jump from one reference node to another.  Some
> people will call this ground bounce but I prefer to think of it as poorly
> referenced signals.
>
> I hope this does not get you into trouble with your analog engineer.  These
> are basic power integrity concepts that need to be resolved anytime we talk
> about power quality and signal referencing.  Any attempt to get S parameters
> for ground will simply lead to confusion and trouble.
>
> Regards,
> Larry Smith
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Wednesday, June 01, 2011 4:34 AM
> To: Rajan Hansa
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Seperate Sparameter for gnd & supply plane !!
>
> Don't suppose.  Analyze.  Your real problem is that you analog engineer
> wants to understand the extent of noise disturbance that is associated
> with the interconnects and PDN.  He first needs to define his problem by
> deciding for each victim what location in the assembly constitutes his
> reference.  Once that is done, the problem can be solved with analytical
> tools.  An equipotential if it exists somewhere convenient reduces the
> effort required to get a reasonable answer.
> Steve
> On 6/1/2011 4:23 AM, Rajan Hansa wrote:
>>
>> Steve,
>>
>> The board has many chips including ours and they all have different
>> current requirement so it won't be correct to consider it
>> equipotential and if I suppose consider them quasi-equipotential, what
>> extra things I have to take care while generating s-parameter ?? OR do
>> you mean to say for quasi-equipotential, separate vdd & gnd
>> s-parameters will take care our problem ?
>>
>> Rajan
>>
>> On Wed, Jun 1, 2011 at 4:10 PM, steve weir <weirsi@xxxxxxxxxx
>> <mailto:weirsi@xxxxxxxxxx>> wrote:
>>
>>     You have a matrix of current sources attached to a matrix of RLCG
>>     legs, and you want to know the noise voltage that occurs between
>>     different point pairs in that matrix.  If the product of the
>>     static and dynamic currents against a section of those legs is
>>     very small, you can treat that region as a quasi- equipotential.
>>     Otherwise you have to treat the point pairs as unique.
>>
>>     Steve
>>
>>
>>     On 6/1/2011 3:29 AM, Rajan Hansa wrote:
>>>
>>>     Steve,
>>>
>>>     You are right !! Our analog engineer basically wants to
>>>     see ground bounce w.r.t pcb ground which was not possible with
>>>     single s-parameter but you said that I have to do whole lot more
>>>     than just produce s-parameters between one port inside the
>>>     package and some other virtual port.
>>>
>>>     Can you help me to understand that why separate s-parameter is
>>>     not sufficient to see ground bounce and what is the right way to
>>>     handle such problems ??
>>>     Rajan
>>>
>>>
>>>     On Wed, Jun 1, 2011 at 3:30 PM, steve weir <weirsi@xxxxxxxxxx
>>>     <mailto:weirsi@xxxxxxxxxx>> wrote:
>>>
>>>         What it sounds like your analog engineer wants is to be able
>>>         to figure out how much bounce will occur in each plane
>>>         relative to some external reference, such as the PCB Vss
>>>         plane.  In order to figure that out, you have to do a whole
>>>         lot more than just produce s-parameters between one port
>>>         inside the package and some other virtual port.  Your analog
>>>         engineer needs to be more specific about what he wants to
>>>         figure out.
>>>
>>>         Steve.
>>>
>>>
>>>         On 6/1/2011 1:41 AM, Rajan Hansa wrote:
>>>
>>>             Guys,
>>>             I have got a request to provide sparameter for one
>>>             package but this time my
>>>             analog engineer wants seperate s-parameter for gnd&
>>>              supply plane. The
>>>             reason he gave is that with single sparameter for vdd&
>>>              gnd plane. He can't
>>>             see gnd noise as s-parameter is generated with gnd as
>>>             reference so it'll
>>>             always be seen as '0' but that problem won't come in
>>>             separate s-parameters
>>>             for gnd&  supply planes.
>>>
>>>             Though the tool I am using has an option to generate
>>>             sparameter with
>>>             reference to some virtual point and I can use that option
>>>             to generate
>>>             separate sparameters for gnd&  supply but problem is that
>>>             I have not still
>>>             understood if using separate s-parameter is a right way
>>>             to do things and if
>>>             it's the standard practice in industry.
>>>
>>>
>>>             Rajan
>>>
>>>
>>>
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>>>         --
>>>         Steve Weir
>>>         IPBLOX, LLC
>>>         150 N. Center St. #211
>>>         Reno, NV  89501
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>>>
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>>>         (707) 780-1951 Fax
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>>>
>>>
>>
>>
>>     --
>>     Steve Weir
>>     IPBLOX, LLC
>>     150 N. Center St. #211
>>     Reno, NV  89501
>>     www.ipblox.com  <http://www.ipblox.com/>
>>
>>     (866) 675-4630 Toll-free
>>     (707) 780-1951 Fax
>>
>>
>>
>
>
> --
> Steve Weir
> IPBLOX, LLC
> 150 N. Center St. #211
> Reno, NV  89501
> www.ipblox.com
>
> (866) 675-4630 Toll-free
> (707) 780-1951 Fax
>
>
>
>
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