# [SI-LIST] Re: SSO pushout, ground bounce definition

*From*: Larry Smith <ldsmith@xxxxxxxxxxxxxxxxxx>*To*: ldsmith@xxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx,Pat_Diao@xxxxxxxx*Date*: Fri, 19 Apr 2002 09:30:49 -0700 (PDT)

Pat - Different EM extractors use different techniques for finding partial inductance and mutuals. A long time ago in their 3D extractor, Ansoft used to sum up the energy in all of the tetrahedron when 1 amp is forced through a conductor. Energy is half of inductance times the current squared, W=L*I^2/2. From the energy and 1 amp, they found inductance. I think the equations that you have quoted below are equivalent, but perhaps easier computationally. Lot's of 2d extractors find the capacitance between structures and calculate inductance from velocity=sqrt(1/LC). Velocity is known from the dielectric constant and the speed of light. As you can see, these extractors do not calculate L directly but infer it from some other quantity. Other 3d extractors descritize conductors up into lots of little rectangular bars. Grover published inductances for simple bar geometries long ago, I think the book has been out of print for more than 50 years. Fundamentally, these inductance extractors are based on Grover's calculations for inductance. There are probably a dozen different methods for EM extraction. To answer your question, yes the several different techniques may obtain different partial inductances for the same geometry problem. The example I gave several emails ago was a bit extreme, I don't think the extracted partial inductances will differ by a factor of 10. But even if they did, they could all produce the same loop inductance and therefor the same results in a circuit simulator. regards, Larry Smith Sun Microsystems > From: Pat Diao <Pat_Diao@xxxxxxxx> > To: "'Larry Smith'" <ldsmith@xxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx > Subject: RE: [SI-LIST] Re: SSO pushout, ground bounce definition > Date: Wed, 17 Apr 2002 17:19:53 -0700 > MIME-Version: 1.0 > > Hi Larry, > > Thanks for the message. The issue regarding inductance is becoming clear in > definition, but still confusing in application. I found the following in > Ansoft tech note: > > "AC Inductance: > All current in AC analysis are surface currents. To take this into account, > the system computes the surface magnetic field, H, such that > H = ... > H dot n =0 > At high frequencies, the magnetic field is tangential to the surface of a > good conductor. After solving for the magnetic field, the system computes > the surface current density K: > K = n x H > and the inductance is found using the relationship: > L = (integral over S)(A dot K dV) " > > I think the calculation is sound for partial L. Frankly this is the only way > I can see to calculate the partial L. Then it come back to the question why > different extractors may come up with different partial L? Do they use > radically different ways to figure out Partial L? > > Regards, > Pat > > -----Original Message----- > From: Larry Smith [mailto:ldsmith@xxxxxxxxxxxxxxxxxx] > Sent: Wednesday, April 17, 2002 9:46 AM > To: ldsmith@xxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx; Pat_Diao@xxxxxxxx > Subject: Re: [SI-LIST] Re: SSO pushout, ground bounce definition > > > Pat - thanks for the comment. > > With almost every electromagnetic extractor you have to be thinking > about where the return current is when you set up the problem. Where > is the reference node? This is especially true with Ansoft as they > make it very easy for you to magically source current somewhere in the > problem and then magically sink current somewhere else. Unless you try > really hard to complete the return path, you will obtain a partial > inductance with all the associated problems discussed in previous > emails. > > To get valid circuit simulation results, you have to apply Vdd and > Ground currents to the ports that you assumed when set up the > extraction problem. Said another way, before you run the extraction > problem, you have to anticipate where your return currents will flow in > your simulation. I have seen many cases where SI engineers do Ansoft > extraction for an SSN problem and the only nodes present are signals > and grounds. If they leave out the Vdd path, they have left out half > of the return current path. ...On the bright side, their simulation > results will be within 50% of being correct. > > It has been several years since I actively worked with Ansoft, but I > often advise engineers that do. For power distribution analysis, I > usually tell them to short the Vdd and ground structures on one side of > the problem, probably at the PCB power planes. Then, force and sink > current in close proximity to each other, probably on some chip or > chip-connect structures. That way, current is forced to run in a > loop. Take the loop inductance number that you get from that > extraction, divide by 2 and call it the "half loop inductance". In the > circuit simulation, assign a half loop inductance to Vdd and another > half loop inductance to ground. I have no idea what the partial or > mutual inductances are for that problem, nor do I care. They don't > mean anything anyway. > > I see a lot of guys draw up the ground structures and extract a partial > inductance. Then they draw up the Vdd structures and extract a partial > inductance. Then they combine the the two partial inductances in the > same circuit simulation problem (with no mutuals) and obtain very > incorrect results. To get correct results, you have to set up the > extraction problem the way you are going to simulate it and simulate > the extracted matrix the way you set it up in extraction. User > beware! > > For the SSN problem, you almost have to draw the Vdd, Ground and signal > structures all at the same time. You then need input and output ports > for each power, ground and signal structure. The path that return > current takes through the structure depends greatly upon the data > patterns given to the drivers in circuit simulation. > > Your second question has to do with the sign polarity on the mutual > inductance. Many extractors will place a minus sign in front of the > mutual inductance, in that case, you want to add it. This question is > closely related to where you put the dots on a transformer. From a > physical perspective, if currents are running parallel (the same > direction) as each other, the flux and mutuals are going to add to the > loop inductance. If currents are running opposite directions to each > other, the flux and mutuals are going to subtract from the loop > inductance. > > regards, > Larry Smith > Sun Microsystems > > > Delivered-To: si-list@xxxxxxxxxxxxx > > From: Pat Diao <Pat_Diao@xxxxxxxx> > > To: "'ldsmith@xxxxxxxxxxxxxxxxxx'" <ldsmith@xxxxxxxxxxxxxxxxxx>, > si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: SSO pushout, ground bounce definition > > Date: Tue, 16 Apr 2002 16:51:09 -0700 > > MIME-Version: 1.0 > > Content-Transfer-Encoding: 8bit > > X-archive-position: 2675 > > X-ecartis-version: Ecartis v1.0.0 > > X-original-sender: Pat_Diao@xxxxxxxx > > X-list: si-list > > > > > > Hi Larry, > > > > Very nice writing about the partial and loop inductance. I agree that a > > partial inductance is meaningless without defining the loop. However, > there > > are two practical questions remaining: > > > > 1. Most of the RLC extractors, including Ansoft Q3D which I use, only > > require users to define the source and sink locations on the net. You can > > draw a ground plane but it is not required, and sometimes there is no > ground > > plane nearby. It then calculates the inductance of the net, which is of > > course the "partial inductance". But the question is, where is the other > > portion of the "loop"? There could be a hundred different ways to define > > the other portion of the loop. Then will we have a hundred different > > inductance values for the net? > > > > 2. On the loop inductance, there is no question about > > L(loop) = L1 + L2 - 2*M12 > > But occasionally one can see > > L(loop) = L1 + L2 + 2*M12 > > Is this a totally wrong equation or the mutual inductance can be of > > different signs? > > > > Thanks, > > Pat > > > > > > > > -----Original Message----- > > From: Larry Smith [mailto:ldsmith@xxxxxxxxxxxxxxxxxx] > > Sent: Tuesday, April 16, 2002 1:56 PM > > To: si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: SSO pushout, ground bounce definition > > > > > > > > Hmmm. Looks like my table separated by tabs did not make it... > > I'll try it again. The table in the text has been replaced. > > > > ------------- Begin Forwarded Message ------------- > > > > Ege - I don't believe there is any contridiction between non-unique > > inductance matricies and unique system voltages. Partial inductances > > are valid as long as they are defined in the context of a loop. > > Partial inductances by themselves have no meaning. But in an > > inductance matrix, where they are combined with all partials and > > mutuals in a loop, the extracted values (not unique) can be used to > > find unique voltages and currents within that loop. > > > > Take the example of two connector pins, one used for a signal and the > > other used for ground. Loop inductance in this case is L1+L2-2*M12 > > (the sum of the self inductances minus two times the mutual inductance > > between them). The following partial and mutual inductances might be > > obtained from two different electromagnetic extractors: > > > > extractor L1 L2 M12 Loop L units > > --------- ----- ----- ----- ------ ----- > > 1 10 10 9 2 nH > > 2 2 2 1 2 nH > > > > Which extractor got the right answer? They both did. Partial and > > mutual inductances are meaningless outside the context of a loop. Both > > sets of extracted data give the same loop inductances and will lead to > > unique voltages and currents when the loop is simulated in a circuit > > analysis tool, even though the extracted partial and mutual inductances > > are not unique. =20 > > > > I see a lot of engineers try to use partial inductance (alone) in a > > circuit simulation. This is at best misleading and may give flat out > > wrong answers in the simulation. I don't think you will find any > > disagreement between Al Ruehli, Bryan Young or any of the other experts > > in this matter (please comment if I have misspoken). > > > > Now concerning measurements, simulation, SI and EMI... For SI > > purposes, it only makes sense to measure voltages in very local areas > > with a probe that has a very short ground lead. When we measure a > > signal with respect to (WRT) local ground, we have measured a > > "difference" voltage. This is a differential measurement, even if it > > is done with a single ended probe. We have not measured the signal WRT > > spice node 0, the center of the earth, or any other place in the > > system. If a simulation is set up correctly, the inductance matrix > > from either extractor 1 or 2 above will deliver an inductance matrix > > that can be simulated and match the hardware measurement of a signal > > WRT it's local ground. > > > > As Raymond Chen has stated in points one and two below, it does not > > make sense to measure anything other voltage WRT local ground. There > > are at least two problems with measurements that are not made WRT local > > ground. If we try to measure across several inches (significant > > portion of a wavelength) we are trying to measure across a time delay. > > What meaning is there in voltage measured across time? If we try to > > measure across a big inductance (i.e. connector pin), magnetic flux > > will penetrate the loop involving the inductance, probe and ground > > lead. The size of the measurement loop will determine the magnetic > > flux "captured" by that loop and lead to non unique measurements which > > depend on where the ground lead is positioned WRT the inductance. With > > probes, the only legitimate thing to measure is the difference between > > a node and it's local reference point, hopefully in an area where the > > time varying magnetic field is not significant. > > > > Even though we don't have a good way to measure "ground bounce" across > > a connector, I believe a circuit simulator with the inductance matrix > > from either extractor 1 or 2 above is capable of simulating the correct > > voltage across the ground pin of a connector (and getting an identical > > voltage solution from either matrix). If 10 signals crossed the > > connector and all switched the same way at the same time, there would > > be a huge (almost Vdd) voltage across the single ground pin. This is > > real and is what we call the SSN or SSO problem. It can cause flipped > > bits and EMI radiation as one local ground gets perturbed WRT to the > > other. Simulations of "ground bounce" are probably more trustworthy > > than measurement, assuming that all partial inductances and mutual > > inductances are given in the context of a loop. It is difficult to > > measure ground bounce directly. The best indicators of ground bounce > > are funny waveforms at the far end of a quiet line and EMI radiation. > > > > regards, > > Larry Smith > > Sun Microsystems > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

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