Larry, In the measurement method you discribed below, you actually measured the total loop inductance ( L_cap+L_vias+L_plane), is it right? There should are some ways to seperate the L_vias and L_plane from the L_totlal, since L_cap is more meaningful for some PDS tools. It is always harder to define/extract partial inductance than loop inductance since loop indutance is always constant. Scott, I like your idea about the "effective inductance curve" related with the location of return current. Did you consider some special cases, such as on a mutil-layer board the plane just below the caps may not be the power/ground plane for this cap? Thanks. Zhiping Scott McMorrow wrote: > Larry, > > No, I didn't send it to the list. Since I hadn't thought about this > much, I wanted to pass > it through you first. I've now forwarded my comments to the list. > > Okay, I see your "effective inductance" approach. It is the self > inductance of the > capacitor minus the mutual inductance relative to other structures. > This effective > inductance will be different with each mounting and measuring method. > This most > dominant factor will be the location of the planes in the z-axis > underneath the > capacitor. > > It may be possible to come up with an "effective inductance curve" which > relates > the partial loop inductance of the capacitor with respect to the > distance it is from > the local planes. > > regards, > > scott > > -- > Scott McMorrow > Principal Engineer > SiQual Interconnect Engineering > 18735 SW Boones Ferry Road > Tualatin, OR 97062-3090 > (503) 239-4400 > http://www.siqual.com > > Larry Smith wrote: > > >Scott - Your comments are 'right on the money'. When we mount > >the capacitor directly above and close to a power plane, the > >loop inductance is much smaller than when it is mounted on 50 ohm > >traces. > > > >Did you mean to send this to SI-list? I think it just came to me. > >I think it would be good if the whole list saw this note. > > > >I like to measure the inductance of the cap mounted on the structure, > >measure the structure itself, subtract and find the "effective inductance" > >of the cap by itself. > > > >regards, > >Larry > > > >>Delivered-To: fixup-ldsmith@xxxxxxxxxxxxxxxxxx@fixme > >>Date: Mon, 18 Mar 2002 14:14:33 -0800 > >>From: Scott McMorrow <scott@xxxxxxxxxx> > >>User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:0.9.8) > >> > >Gecko/20020204 > > > >>X-Accept-Language: en-us > >>MIME-Version: 1.0 > >>To: ldsmith@xxxxxxxxxxxxxxxxxx > >>Subject: Re: [SI-LIST] Re: Importance of Package Height > >>Content-Transfer-Encoding: 7bit > >> > >>Larry, > >> > >>It seems to me that the inductance of a capacitor would be heavily > >>influenced by the > >>placement of a return plane below the capacitor. The mutual inductance > >>between the > >>capacitor and the plane would significantly lower the loop inductance. > >> Is this possibly > >>what is happening when capacitors are mounted on low impedance power planes? > >>Here we have mutual inductance cancellation between the planes and > >>mutual cancelation > >>between the capacitor body and the closest plane. > >> > >>The problem we have is that for a capacitor the inductance is not > >>defined until the loop > >>is closed. The partial inductance of the capacitor body is heavily > >>influenced by the > >>method of measurement and the insertion into the plane. I am not sure > >>that there > >>is one method of measurement which can be used to extract the parameters > >>for > >>a capacitor, any more than there is one method which can be used to > >>extract parameters > >>for card edge connectors. In both cases, the loop inductance is > >>dependant upon the > >>the mutual inductance between the component and the underlying plane and > >>the > >>direction of return current flow through the plane. > >> > >>For the case of a measurement where a VNA is used with 50ohm launches > >>into the > >>capacitor, there is no return current on any plane and therefore, no > >>mutual cancelation. > >> > >>For the case of a measurement in a loaded planar board, there is return > >>current on the > >>plane and and associated mutual cancelation which is beneficial to loop > >>inductance. > >> > >>It might be possible to perform some theoretical modeling of a > >>simplified capacitor > >>structure with different measurment configurations using a 3D field > >>solver to extract > >>partial inductances and come up with some more general conclusions. My > >>guess is that > >>there will not be one inductance number that can be used for a specific > >>capacitor, since > >>it is such a poorly referenced structure in the first place. But, there > >>is probably a range > >>of inductances which are dependent upon the placement of planes in a > >>board and whether > >>or not currents flow between the capacitor and the plane. > >> > >>Since inductance is only defined around a loop, the only valid > >>measurement is within > >>the same structure where the device will be applied, so that all mutuals > >>are correctly > >>accounted for. > >> > >>regards, > >> > >>scott > >> > >> > >>-- > >>Scott McMorrow > >>Principal Engineer > >>SiQual Interconnect Engineering > >>18735 SW Boones Ferry Road > >>Tualatin, OR 97062-3090 > >>(503) 239-4400 > >>http://www.siqual.com > >> > >> > >> > >>>It turns out that the resonant frequency of a capacitor mounted on low > >>>impedance (50 mOhm or so) power planes is higher than the same > >>>capacitor mounted on a 50 Ohm transmission line. Therefor, the ESL is > >>>lower for the cap on power planes than it is for a cap on 50 Ohm > >>>traces. The physical reason has a lot to do with the resonant > >>>structure for each measurement. The most meaningful measurments of > >>>capacitors intended for decoupling will be made on low impedance (low > >>>inductance) mounting structures. > >>> > >>>regards, > >>>Larry Smith, > >>>Sun Microsystems > >>> > >>>>Date: Mon, 18 Mar 2002 12:44:35 -0800 > >>>>From: Zhiping Yang <zhiping@xxxxxxxxx> > >>>>X-Accept-Language: en > >>>>MIME-Version: 1.0 > >>>>To: ldsmith@xxxxxxxxxxxxxxxxxx > >>>>CC: si-list@xxxxxxxxxxxxx, ARIAZI@xxxxxxxxxxx > >>>>Subject: Re: [SI-LIST] Re: Importance of Package Height > >>>>Content-Transfer-Encoding: 7bit > >>>> > >>>>Larry, > >>>> > >>>>Thanks for sharing your ideas. > >>>> > >>>>Can you explain why the measurement from direct VNA with 50Ohm fixture > >>>>is greater than on PCB? > >>>> > >>>>Here are my understandings: > >>>>1. VNA with 50Ohm fixtures (Calibrate VNA to the cap pads, can we do this > >>>> in lab? I am not positive.) > >>>> L_measure=L_cap_self_inductance > >>>>2. VNA with 50Ohm fixtures (VNA is not calibrated) > >>>> L_meausre=L_cap_self_indutnace+L_fixtures+M_mutual > >>>>3. Measurement with cap on PCB structures > >>>> L_meaure=L_cap_self_inductance+L_vias+L_plane+M_mutuals > >>>> > >>>>For my understanding, case 1 gives the smallest inductance. Maybe you > >>>>are refering the VNA measurement to case 2. > >>>> > >>>>For the case 3, the results are also heavily depended on cap layout > >>>>and PCB stackup. How can you build the universal cap component > >>>>library for different PCBs? > >>>> > >>>>Here is my thought. When you build the cap libraries, the ESL should > >>>>only be self inductance of the cap. The mounting inductance (vias, plane > >>>>and traces) should be compensated in simulation tools. In this way, the > >>>>cap library can be applied to any PCBs. Anyone disagree? > >>>> > >>>>Thanks. > >>>> > >>>>Zhiping > >>>> > >>>> > >>>> > >>>>-- > >>>> Zhiping Yang, Ph. D. > >>>> Hardware Engineer > >>>> Cisco Systems > >>>> 270 West Tasman Drive > >>>> Mail Stop:SJCG/2/2 > >>>> San Jose, CA 95134 | | > >>>> email: zhiping@xxxxxxxxx :|: :|: > >>>> Tel : 408 525 5690 :|||: :|||: > >>>> Fax : 408 526 5504 .:|||||||:..:|||||||:. > >>>>***************************************************** > >>>> > >>>>Larry Smith wrote: > >>>> > >>>>>Abe - As mentioned earlier, the inductance you get depends greatly on > >>>>>the assumptions you make, either in measurement or software > >>>>>extraction. > >>>>> > >>>>>The thing we really want to know is "what is the series resonant > >>>>>frequency of a decoupling capacitor mounted on the pads, vias and power > >>>>>planes of our products?" From the series resonant frequency, we back > >>>>>calculate the ESL. My definition of ESL is the equivalent inductance > >>>>>that causes the capacitor to have the low impedance dip at the > >>>>>frequency that I measure. > >>>>> > >>>>>We find that the ESL obtained from measurement depends greatly on the > >>>>>fixture used to make the measurement. Most people are now using a VNA > >>>>>S21 measurement to characterize capacitors. The purists will make a > >>>>>fixture that has a 'perfect' 50 ohm through impedance and mount the > >>>>>capacitor on that fixture. That S21 measurement gives a clear > >>>>>capacitance, resonance and inductance portion of the curve. Most > >>>>>people will argue that the "ESL" determined by the resonant frequency > >>>>>of that measurement is the "ESL" of the capacitor. Valid argument, but > >>>>>it does not apply to our products. > >>>>> > >>>>>Ok, now go mount that same capacitor on a set of low inductance pads > >>>>>and vias that are connected to thin power planes near the surface of a > >>>>>PCB (my product..). Use the same VNA to make an S21 measurement of the > >>>>>power planes. You will find that the resonant dip is as much as 25% > >>>>>higher in frequency than the cap measured on the 50 Ohm fixture. Back > >>>>>calculate the inductance and you find that the ESL for the capacitor is > >>>>>50% higher on the 50 Ohm fixture than it is on the power planes. > >>>>>Hmmmm. Once again, the ESL of the capacitor depends greatly on the > >>>>>assumptions and fixture that you use when you measure it. > >>>>> > >>>>>The inductances given on both the Kemet and AVX web sites are too high > >>>>>for my purposes. They are not useful for calculating or simulating the > >>>>>series resonant frequency when the capacitor is used for power supply > >>>>>decoupling in our products. I would encourage any SI'ers that are > >>>>>truly interested in power distribution to go repeat some of these > >>>>>measurements on your own products. Find the ESL of a capacitor by > >>>>>measuring the resonant frequency when mounted on low inductance > >>>>>products and fixtures. > >>>>> > >>>>>Sun and Cadence sponsored a capacitor consortium meeting last October > >>>>>which was attended by about half a dozen major capacitor suppliers. We > >>>>>described the software tool techniques that we use to design power > >>>>>distribution systems (Specctra Quest Power Integrity Tool). Very > >>>>>quickly it becomes obvious that software tools for PDS analysis rely > >>>>>heavily on the accuracy of capacitor models. Unfortunately, the > >>>>>standards for measuring capacitor parameters (capacitance, ESR and ESL) > >>>>>do not apply very well for capacitors used for PDS decoupling above > >>>>>about 10 MHz. We found that the capacitor suppliers are very receptive > >>>>>to these ideas and would like to make measurements that are meaningful > >>>>>for their customers. The problem is that there is not a strong > >>>>>consensus (much less a standard) in our industry on what ESL is and how > >>>>>it should be measured. > >>>>> > >>>>>As you know, Sun has been working in this area for some time. We are > >>>>>motivated to share this information with the rest of the industry in > >>>>>hopes that meaningful measurements can be made by the cap vendors, which > >>>>>lead to accurate simulation models for our PDS tools. But, there must > >>>>>be agreement on what ESL is and how to measure it. That's why you get > >>>>>all these loooonng emails from me.. We also publish our data in IEEE > >>>>>conferences to get peer review on these concepts. Eventually, meaningful > >>>>>measurement standards and simulation techniques will come out of this. > >>>>> > >>>>>regards, > >>>>>Larry Smith, > >>>>>Sun Microsystems > >>>>> > >>>>>>From: "Abe Riazi" <ARIAZI@xxxxxxxxxxx> > >>>>>>To: <si-list@xxxxxxxxxxxxx> > >>>>>>Cc: <ldsmith@xxxxxxxxxxxxxxxxxx>, <jrbarnes@xxxxxxxxx> > >>>>>>Subject: Re: [SI-LIST] Re: Importance of Package Height > >>>>>>Date: Sun, 17 Mar 2002 22:52:29 -0800 > >>>>>>MIME-Version: 1.0 > >>>>>>Content-Transfer-Encoding: 7bit > >>>>>>X-Priority: 3 > >>>>>>X-MSMail-Priority: Normal > >>>>>>X-Mimeole: Produced By Microsoft MimeOLE V5.50.4133.2400 > >>>>>> > >>>>>>Larry Smith Wrote: > >>>>>> > >>>>>>>I'm not sure what assumptions Kemet is making for > >>>>>>>inductance in their calculator. Their inductance seems way high to > >>>>>>>me. We have measured a lot of caps from a half dozen vendors and Kemet > >>>>>>>is no different than any of the others. When mounted on a low > >>>>>>>inductance structure, the Kemet cap will behave as if it is > >>>>>>>significantly less than 1 nH. > >>>>>>> > >>>>>>> > >>>>>>Larry, > >>>>>> > >>>>>>Thank you for an instructive reply. > >>>>>> > >>>>>>The AVX SpiCap program may output considerably > >>>>>>smaller ESL for similar capacitor than the kemet Spice Simulator. > >>>>>>As an example, for the ceramic capacitor we recently discussed > >>>>>>the AVX answer is 1.0 nH compared to kemet's 1.94 nH. > >>>>>>Sometimes, use of Kemet software is preferable because it can show > >>>>>>variations of ESL with capacitance values (for fixed package > >>>>>>size) while the AVX calculator can not. > >>>>>> > >>>>>>I think (please correct me if I am wrong) the 1.0 nH by AVX and > >>>>>>the Kemet's 1.94 nH (and all other ESL results by these two > >>>>>>programs) apply to un-mounted capacitors. Therefore, based > >>>>>>on your expected 600 pH for that 100 nF 0805 X7R cap > >>>>>>both of these manufacturer calculators are providing ESL values > >>>>>>which are too large/inaccurate in this case. > >>>>>> > >>>>>>Best Regards, > >>>>>> > >>>>>>Abe Riazi > >>>>>>ServerWorks > >>>>>> > >>>>>>P.S. I also compared the AVX 1.0 nH value to the following measurement > >>>>>>result earlier reported by John Barnes: > >>>>>> > >>>>>> > >>>>>> Nominal > >>>>>> Case Capacitance WV ESL > >>>>>> Size Dielectric (pF) (V) Manufacturer Partnum (pH) > >>>>>>------ ---------- ----------- ---- ---------------------------- -------- > >>>>>> > >>>>>>0805 X7R 100000 25 AVX 08053C104JAT2A 900 > >>>>>> > >>>>>>and found them reasonably close. > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>> > >>>>>------------------------------------------------------------------ > >>>>>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: > >>>>>//www.freelists.org/webpage/si-list > >>>>> > >>>>>For help: > >>>>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >>>>> > >>>>>List archives are viewable at: > >>>>> //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 > >>>>> > >>>------------------------------------------------------------------ > >>>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: > >>>//www.freelists.org/webpage/si-list > >>> > >>>For help: > >>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >>> > >>>List archives are viewable at: > >>> //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 > >>> > >>> > >>> > >>> > >> > > > > > > ------------------------------------------------------------------ > 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: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //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 ------------------------------------------------------------------ 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: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //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