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. > > I've seen a lot of analog designers get really confused about this and try to > 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 > about the po > 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. >>> >>> Experts comments please. >>> >>> Rajan >>> >>> >>> >>> ------------------------------------------------------------------ >>> To unsubscribe from si-list: >>> si-list-request@xxxxxxxxxxxxx >>> <mailto:si-list-request@xxxxxxxxxxxxx> with 'unsubscribe' >>> in the Subject field >>> >>> or to administer your membership from a web page, go to: >>> //www.freelists.org/webpage/si-list >>> >>> For help: >>> si-list-request@xxxxxxxxxxxxx >>> <mailto:si-list-request@xxxxxxxxxxxxx> with 'help' in the >>> Subject field >>> >>> >>> List technical documents are available at: >>> http://www.si-list.net <http://www.si-list.net/> >>> >>> List archives are viewable at: >>> //www.freelists.org/archives/si-list >>> >>> Old (prior to June 6, 2001) list archives are viewable at: >>> http://www.qsl.net/wb6tpu >>> >>> >>> >>> >>> >>> -- >>> Steve Weir >>> IPBLOX, LLC >>> 150 N. Center St. #211 >>> Reno, NV 89501 >>> www.ipblox.com <http://www.ipblox.com/> >>> >>> (775) 299-4236 Business >>> (866) 675-4630 Toll-free >>> (707) 780-1951 Fax >>> >>> >>> >> >> >> -- >> Steve Weir >> IPBLOX, LLC >> 150 N. Center St. #211 >> Reno, NV 89501 >> www.ipblox.com <http://www.ipblox.com/> >> >> (775) 299-4236 Business >> (866) 675-4630 Toll-free >> (707) 780-1951 Fax >> >> >> > > > -- > Steve Weir > IPBLOX, LLC > 150 N. 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