Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
Thanks for the in-depth story and link. Rene Ian Wilson wrote: The first resonance (at least) of a cap is series, so looks like a short circuit. By adding a number of different valued caps you can scatter a number of these nice AC shorts around your board and around your frequencies of interest. [ snip ] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
On 07:14 PM 4/06/2003, John Sheahan said: On Wed, Jun 04, 2003 at 09:34:18AM +0200, Norbert Hoppe wrote: When selecting parallel caps, it is important to remember that as the larger value capacitor goes inductive, the smaller value cap is still capacitive. At a particular frequency, a LC circuit is developed between the 2 caps. An infinite impedance could be generated with no decoupling benefit provided. When this occurs, single-capacitor decoupling is all that one can use for this application. actually - if you look at ESL graphs for multilayer SMD caps - you will see it depends much more on case size than on capacitance. So the 100n tends to win. This quote I think may be older wisdom for thruhole components. john The first resonance (at least) of a cap is series, so looks like a short circuit. By adding a number of different valued caps you can scatter a number of these nice AC shorts around your board and around your frequencies of interest. Above resonance the reactive impedance starts to rise as the impedance characteristic is now inductive. In many cases this is not an issue, as the effective reactance is still low in the frequencies of interest. In other situations, though, it is a critical issue and hence designers have used, and will continue to use, a variety of values in parallel - very common in RF environments. However, big small caps, or is that small big caps, you know ... large capacitance in small volume, have pretty cruddy material, X7R if you are lucky or Z5U if capacitance is big. These materials have pretty poor, and frequency dependent, ESR which decrease their value as decouplers. Due largely to the effects of the lossy material, a Kemet, for example, X7R shows a sloppy self resonance and the following series impedance at 100 MHz: Value Size Impedance 103 0603~1 ohm 103 0805~0.5 ohm 103 1206~0.3 ohm 104 0805~1 ohm 104 1206~1 ohm So if you spec a 10nF 0603 you have a resistor, not a decoupler, at 100 MHz. According to Kemet, the 0603 only performs better than the other sizes at over the narrow freq range of about 10 to 30 MHz. The lossy dielectric, and the need to use thinner metal in the large capacitances (to keep the pkg the same), is killing the performance. Maybe that is an overstatement - but compare the self resonance curves of a COG/NPO material to that of a X7R, or worse Z5U, you can see the dramatic effect the losses have on the resonance shape. COG/NPO has dissipation factors in the order or 0.1% while the other materials are between about 2.5 to 5% or more. (In fact, the lower Q of the high capaciatnce devices is partially a good thing. Having high Q resonances around a board is a shocker when you find you fail EMC.) Note also that the 100n 0805 has roughly *twice* the impedance @ 100 MHz than the cap *one tenth* the value in the same pkg! In this case, if you are operating above 30 MHz the 1206 10n wins, followed closely by the 0805 10n. 100n in any pkg and 10n in 0603 have about twice the impedance. See figures 4, 5 6 of: http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3102Gce/$file/F3102GCe.pdf There is always progress in material science so the small-packaged, larger capacitance devices get better over time. For modern high speed decoupling - I use lots of 10n devices, a few bulk devices and good (hopefully) layer stackup and split plane arrangement. Specific devices operating at speed will have special treatment. Currently, I side with the get to the plane fast crowd and have my supply via close to the power pads and then decoupling caps strung to these same vias with nice fat tracks. I don't, usually, have a via, track, cap, then component pad arrangement - though my guess is, with the right sort of component selection either arrangement can be done well. Ian Wilson * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
Ian: Thanks for the refresher lesson in how real world parts you can actually get your hands on perform as opposed to theoretically, a smaller case size will always perform better. Best regards, Ivan Baggett Bagotronix Inc. website: www.bagotronix.com - Original Message - From: Ian Wilson [EMAIL PROTECTED] To: Protel EDA Forum [EMAIL PROTECTED] Sent: Wednesday, June 04, 2003 7:37 AM Subject: Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance On 07:14 PM 4/06/2003, John Sheahan said: On Wed, Jun 04, 2003 at 09:34:18AM +0200, Norbert Hoppe wrote: When selecting parallel caps, it is important to remember that as the larger value capacitor goes inductive, the smaller value cap is still capacitive. At a particular frequency, a LC circuit is developed between the 2 caps. An infinite impedance could be generated with no decoupling benefit provided. When this occurs, single-capacitor decoupling is all that one can use for this application. actually - if you look at ESL graphs for multilayer SMD caps - you will see it depends much more on case size than on capacitance. So the 100n tends to win. This quote I think may be older wisdom for thruhole components. john The first resonance (at least) of a cap is series, so looks like a short circuit. By adding a number of different valued caps you can scatter a number of these nice AC shorts around your board and around your frequencies of interest. Above resonance the reactive impedance starts to rise as the impedance characteristic is now inductive. In many cases this is not an issue, as the effective reactance is still low in the frequencies of interest. In other situations, though, it is a critical issue and hence designers have used, and will continue to use, a variety of values in parallel - very common in RF environments. However, big small caps, or is that small big caps, you know ... large capacitance in small volume, have pretty cruddy material, X7R if you are lucky or Z5U if capacitance is big. These materials have pretty poor, and frequency dependent, ESR which decrease their value as decouplers. Due largely to the effects of the lossy material, a Kemet, for example, X7R shows a sloppy self resonance and the following series impedance at 100 MHz: Value Size Impedance 103 0603~1 ohm 103 0805~0.5 ohm 103 1206~0.3 ohm 104 0805~1 ohm 104 1206~1 ohm So if you spec a 10nF 0603 you have a resistor, not a decoupler, at 100 MHz. According to Kemet, the 0603 only performs better than the other sizes at over the narrow freq range of about 10 to 30 MHz. The lossy dielectric, and the need to use thinner metal in the large capacitances (to keep the pkg the same), is killing the performance. Maybe that is an overstatement - but compare the self resonance curves of a COG/NPO material to that of a X7R, or worse Z5U, you can see the dramatic effect the losses have on the resonance shape. COG/NPO has dissipation factors in the order or 0.1% while the other materials are between about 2.5 to 5% or more. (In fact, the lower Q of the high capaciatnce devices is partially a good thing. Having high Q resonances around a board is a shocker when you find you fail EMC.) Note also that the 100n 0805 has roughly *twice* the impedance @ 100 MHz than the cap *one tenth* the value in the same pkg! In this case, if you are operating above 30 MHz the 1206 10n wins, followed closely by the 0805 10n. 100n in any pkg and 10n in 0603 have about twice the impedance. See figures 4, 5 6 of: http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3102Gce/$fil e/F3102GCe.pdf There is always progress in material science so the small-packaged, larger capacitance devices get better over time. For modern high speed decoupling - I use lots of 10n devices, a few bulk devices and good (hopefully) layer stackup and split plane arrangement. Specific devices operating at speed will have special treatment. Currently, I side with the get to the plane fast crowd and have my supply via close to the power pads and then decoupling caps strung to these same vias with nice fat tracks. I don't, usually, have a via, track, cap, then component pad arrangement - though my guess is, with the right sort of component selection either arrangement can be done well. Ian Wilson * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
On Wed, Jun 04, 2003 at 09:37:19PM +1000, Ian Wilson wrote: Thanks for the well reasoned response Ian. I went through this a few months ago, but needed 0402 package (was a dense board) and a similar line of research showed 100n was the best choice there. But it depends on the particular caps chosen. Sun publishes some of the best SI reports in this area I have found particularly covering the concept of (maesured) low plane impedance over frequency. see for example http://groups.yahoo.com/group/si-list/files/Signal%20Integrity%20Documents/Published%20SI%20Papers%20from%20Sun/ Minor quibbles with some of the details inline On 07:14 PM 4/06/2003, John Sheahan said: On Wed, Jun 04, 2003 at 09:34:18AM +0200, Norbert Hoppe wrote: When selecting parallel caps, it is important to remember that as the larger value capacitor goes inductive, the smaller value cap is still capacitive. At a particular frequency, a LC circuit is developed between the 2 caps. An infinite impedance could be generated with no decoupling benefit provided. When this occurs, single-capacitor decoupling is all that one can use for this application. actually - if you look at ESL graphs for multilayer SMD caps - you will see it depends much more on case size than on capacitance. So the 100n tends to win. This quote I think may be older wisdom for thruhole components. john The first resonance (at least) of a cap is series, so looks like a short circuit. By adding a number of different valued caps you can scatter a number of these nice AC shorts around your board and around your frequencies of interest. unfortunatley, these resonant circuits can sometimes clobber each other when there are different values involved. But yes - that series resonance helps a lot. Above resonance the reactive impedance starts to rise as the impedance characteristic is now inductive. In many cases this is not an issue, as the effective reactance is still low in the frequencies of interest. In other situations, though, it is a critical issue and hence designers have used, and will continue to use, a variety of values in parallel - very common in RF environments. sure, we want nice low ESL and ESR values here. The capacitance value (decoupler value) is really useful only below the series resonance point, which also may matter. And that is a big chunk of spectrun from the bulk cap and maybe PSU up to say 100KHz region, thru to the 100M range. However, big small caps, or is that small big caps, you know ... large capacitance in small volume, have pretty cruddy material, X7R if you are lucky or Z5U if capacitance is big. These materials have pretty poor, and frequency dependent, ESR which decrease their value as decouplers. Due largely to the effects of the lossy material, a Kemet, for example, X7R shows a sloppy self resonance and the following series impedance at 100 MHz: Value Size Impedance 103 0603~1 ohm 103 0805~0.5 ohm 103 1206~0.3 ohm 104 0805~1 ohm 104 1206~1 ohm So if you spec a 10nF 0603 you have a resistor, not a decoupler, at 100 MHz. probably only a NPO lower than 1n and the interplane capacitance is working here. The 100n/10n argument is null as we are essentially at/past series resonance. Ovviously here we have an L or an R , not a C. But we do have a low value L or R so we are still decoupling, - the point of the dcoupler is to attenuate noise. C is just the way we often tend to do this. A sprinkling of 10-100pF value caps appeal to some in the 100Mhz range. According to Kemet, the 0603 only performs better than the other sizes at over the narrow freq range of about 10 to 30 MHz. The lossy dielectric, and the need to use thinner metal in the large capacitances (to keep the pkg the same), is killing the performance. Maybe that is an overstatement - but compare the self resonance curves of a COG/NPO material to that of a X7R, or worse Z5U, you can see the dramatic effect the losses have on the resonance shape. COG/NPO has dissipation factors in the order or 0.1% while the other materials are between about 2.5 to 5% or more. (In fact, the lower Q of the high capaciatnce devices is partially a good thing. Having high Q resonances around a board is a shocker when you find you fail EMC.) agree Note also that the 100n 0805 has roughly *twice* the impedance @ 100 MHz than the cap *one tenth* the value in the same pkg! In this case, if you are operating above 30 MHz the 1206 10n wins, followed closely by the 0805 10n. 100n in any pkg and 10n in 0603 have about twice the impedance. here you would do better with 1n NPO however.. See figures 4, 5 6 of: http://www.kemet.com/kemet/web/homepage/kechome.nsf/vapubfiles/F3102Gce/$file/F3102GCe.pdf There is always progress in material science so the small-packaged, larger capacitance devices get better over time. For
Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
On 08:28 AM 5/06/2003, John Sheahan said: snip.. Value Size Impedance 103 0603~1 ohm 103 0805~0.5 ohm 103 1206~0.3 ohm 104 0805~1 ohm 104 1206~1 ohm So if you spec a 10nF 0603 you have a resistor, not a decoupler, at 100 MHz. probably only a NPO lower than 1n and the interplane capacitance is working here. The 100n/10n argument is null as we are essentially at/past series resonance. Ovviously here we have an L or an R , not a C. But we do have a low value L or R so we are still decoupling, - the point of the dcoupler is to attenuate noise. C is just the way we often tend to do this. A sprinkling of 10-100pF value caps appeal to some in the 100Mhz range. I beg to disagree a little - the interplane cap maybe, but insofar as the bulk C's ... I am not looking at theoretical impedance (1/jwC) rather the published impedance curves from the manufacturer. Operating past resonance is nothing special, and not necessarily a problem. All you need to worry about is minimising the AC impedance between the power nets over a suitably broad range of frequencies and to an adequately low level - both application dependent. Operating past resonance simply means the impedance is inductive and rising with freq - so what, as long as it is low enough, that is generally all you need to know. (One big issue here is production spread of course - self resonance varies widely from device to device. If your circuit relies on self resonance to null a specific noise spike, and you are using high-Q caps to do this, you may have problems in production. I can imagine what the production team would think about trimmable decoupling caps. :-) An NPO is only available in small cap values and so will be operating as a pretty good cap at low freqs and hence fairly ineffective. At 30 MHz the Kemet 1nF 1206 NPO is speced at typically 2 to 3 ohms. Even if you had 1nF of high Q interplane capacitance (which you won't - see next para) you would still only have an AC impedance of 1.6 ohms. 0.2A switching current - 0.3 V ripple. I would therefore look at the published impedance curves and find the most appropriate set of caps that supplement the plane capacitance at the dominant freqs of interest. Forget the cap value - look at the impedance curve - for high speed decoupling situations this is all that matters. 1nF of *high-Q* interplane capacitance! Not likely, even if the parallel plate capacitance equation gave that much in practice the resistance and inductance of the plane copper, and very uneven current distribution will mean that much of this theoretical capacitance is out-of-circuit for a particular component supply pin. At higher frequencies you also have to consider the effects of propagation delay - some of the capacitance is too far away for the electrons to whizz over and smooth the voltage ripple due to the current spike - 1ns is about 180 cm assuming a 0.6 velocity factor (conservative as the Er is likely to be such that the velocity factor is lower (1/sqrt(Er)). Even if I had superconducting power planes I would still have an effective area that is dependent on the speed of the transistions. ..snip.. Note also that the 100n 0805 has roughly *twice* the impedance @ 100 MHz than the cap *one tenth* the value in the same pkg! In this case, if you are operating above 30 MHz the 1206 10n wins, followed closely by the 0805 10n. 100n in any pkg and 10n in 0603 have about twice the impedance. here you would do better with 1n NPO however.. In a 1206 though, then you potentially get into the issue of longer tracks to the component pad so more inductance, and the 1206 1n NPO only performs better at 100MHz and a small range either side - due to the high Q the resonance is sharp. So you have poor decoupling at 30 MHz - hence the need to parallel a big and sloppy with a sharp and quick. Due to the series elements (look at the cap model) the interactions between caps is not all that much, when one is looking like a high impedance the other is becoming a low impedance - a high impedance in parallel with a low is a low.. Take the published cap models and do some SPICE sims - it is easy to see the results. Better still use real caps and a spec-an/tracking generator or VNA. The above is my, possibly flawed analysis...I suspect that is enough from me on this, Hooroo, Ian * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
On Thu, Jun 05, 2003 at 10:29:29AM +1000, Ian Wilson wrote: about is minimising the AC impedance between the power nets over a suitably broad range of frequencies and to an adequately low level - both application dependent. Operating past resonance simply means the impedance is inductive and rising with freq - so what, as long as it is low enough, that is generally all you need to know. yes - just as long as we are not getting too near the parallel resonance. A Q issue as you point out. may have problems in production. I can imagine what the production team would think about trimmable decoupling caps. :-) select on test decouplers I have yet to resort to fortunately :) 1nF of *high-Q* interplane capacitance! Not likely, even if the parallel sorry if I was unclear. I did not mean to suggest the board planes were this good. 100p ~ 1N NPO may win in a brief band somewhere between 100M and 1G. The plane cap helps a bit there and above. 100n or 10n is more useful at lower frequencies, - a NPO does not eliminate them - but sometimes can help the 10n/100n's a little further away to be useful. that much, when one is looking like a high impedance the other is becoming a low impedance - a high impedance in parallel with a low is a low.. Take the published cap models and do some SPICE sims - it is easy to see the results. Better still use real caps and a spec-an/tracking generator or VNA. I think the VNA is the tool of choice for designing this stuff. A spec an can tell you how much noise you have got - but its tough to be analytical when the noise injection model from a FPGA say is so lacking. john * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
[PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
JH 2/ Use a spread of capacitor values so that you swap one or two deep JH resonant nulls for a swag of shallower ones spread across the spectrum. I found this to be interesting. I've just about finished reading Digital Design for Interference Specifications David l. Terrell, R.Kenneth Keenan, 1997, Published by NewNes ISBN 0-7506-7282-X. The authors seem to take the opposite position that (bulk capacitance aside) using the same value bypass caps lowers the overall ESR, which (they say) results in a lower overall spectrum. And that mixing (for example) .1uf and .01uf values is a bad idea. In any case, it was an interesting read. I'd be curious if anyone else has views on this subject, one way or the other.. (The book also has some layer stackup advice.) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance
- Original Message - From: Phillip Stevens [EMAIL PROTECTED] To: Protel EDA Forum [EMAIL PROTECTED] Sent: Wednesday, June 04, 2003 7:22 AM Subject: [PEDA] Re[2]: six or eight-layer (or more?) stackups - Capacitance JH 2/ Use a spread of capacitor values so that you swap one or two deep JH resonant nulls for a swag of shallower ones spread across the spectrum. I found this to be interesting. I've just about finished reading Digital Design for Interference Specifications David l. Terrell, R.Kenneth Keenan, 1997, Published by NewNes ISBN 0-7506-7282-X. The authors seem to take the opposite position that (bulk capacitance aside) using the same value bypass caps lowers the overall ESR, which (they say) results in a lower overall spectrum. And that mixing (for example) .1uf and .01uf values is a bad idea. In any case, it was an interesting read. I'd be curious if anyone else has views on this subject, one way or the other.. The same advice comes from the book Montrose: Printed Circuit Board Design Techniques quote When selecting parallel caps, it is important to remember that as the larger value capacitor goes inductive, the smaller value cap is still capacitive. At a particular frequency, a LC circuit is developed between the 2 caps. An infinite impedance could be generated with no decoupling benefit provided. When this occurs, single-capacitor decoupling is all that one can use for this application. /quote Regards, Norbert. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
