**From:** *[email protected]*

**Date:** Wed Mar 14 2001 - 14:56:16 PST

**Next message:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Previous message:**Jian X. Zheng: "RE: [SI-LIST] : Re: approximations for partial self inductance"**Maybe in reply to:**Sainath Nimmagadda: "[SI-LIST] : Re: approximations for partial self inductance"**Next in thread:**Reid, Chris: "RE: [SI-LIST] : Re: approximations for partial self inductance"

We need to be careful in our comments. People seem to be mixing situations.

This is dangerous when the starting point is a discussion of approximations.

The original question was about vias for decoupling, not signal vias. Yes

Jian, you are quite correct that there are many cases where a capacitor

models the effect of a via on a SIGNAL better than an inductor.

I have never seen a via through a multilayer board that looked like a long

straight round wire; a via has pads on every layer which add capacitance.

So let me highlight two key parts of Brian's comments

*> >>When ***d>>r***, the external partial
*

*> >>self-inductance of a straight round wire is
*

*> >>L=5.08d*{ln(2d/r +/- whatever
*

*> >>This formula **should not be used** for vias because it assumes that
*

*> >>the length is much greater than the diameter.
*

For DECOUPLING, considering a via alone is risky; one should consider the

LOOP defined by the Power/Ground via pairs.

Aubrey Sparkman

Signal Integrity

[email protected]

(512) 723-3592

*> -----Original Message-----
*

*> From: Jian X. Zheng [mailto:[email protected]]
*

*> Sent: Wednesday, March 14, 2001 1:44 PM
*

*> To: Howard Johnson; [email protected]
*

*> Subject: RE: [SI-LIST] : Re: approximations for partial self
*

*> inductance
*

*>
*

*>
*

*> Hi, Howard:
*

*>
*

*> I like your comment on via inductance.
*

*>
*

*> -------------------------------------------------------
*

*> For a signal which pops from one side of the
*

*> plane, through a via, to the opposite side
*

*> of the same plane (i.e., the return current
*

*> doesn't have to jump planes), the via
*

*> inductance is very, very low. This is a best-case
*

*> scenario. I don't know a good way to make this
*

*> calculation except with a true 3-D E&M field solver.
*

*> -------------------------------------------------------
*

*>
*

*> Many people consider a via can be approximated by an
*

*> inductance. However, I
*

*> believe it might not be a very good approximation. It does
*

*> seem to me the
*

*> via inductance can be quite low and there might be
*

*> significant capacitance
*

*> involved. I can use our full wave electromagnetic simulator
*

*> IE3D to confirm
*

*> it in seconds or minutes.
*

*>
*

*> I published a paper on the IEEE Trans. on Antennas and
*

*> Propag. vol. 39, No.1
*

*> January. The paper is on a coaxial fed probe (or via). The
*

*> structure is not
*

*> exactly the same as in the multiple layered board. However, it is very
*

*> similar. Accurate L and C can be obtained analytically. It is quite
*

*> complicated and I would not put it here. The interesting
*

*> thing is that the L
*

*> and C involved have the frequency dependency of log(f).
*

*> Interested users can
*

*> find the formulas from the literature. Thanks!
*

*>
*

*> --------------------------------------------------------------
*

*> ---------
*

*> Jian-X. Zheng, Ph.D
*

*> Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA
*

*> 94538, U.S.A.
*

*> Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
*

*> ---------------------------------------------------------------------
*

*>
*

*> > -----Original Message-----
*

*> > From: [email protected]
*

*> > [mailto:[email protected]]On Behalf Of Howard Johnson
*

*> > Sent: Wednesday, March 14, 2001 10:22 AM
*

*> > To: [email protected]
*

*> > Subject: [SI-LIST] : Re: approximations for partial self inductance
*

*> >
*

*> >
*

*> > Dear Itzhak Hirshtal and Brian Young,
*

*> >
*

*> > The difficulties with approximating the inductance
*

*> > of a via are even worse than you
*

*> > may have suspected. Both approximations are flawed whether
*

*> > you use +1 or -3/4, (or, as I have also seen, -1).
*

*> >
*

*> > The issue of the exact constant (1, -3/4, or something
*

*> > else) depends critically on your assumption about
*

*> > the path of returning signal current. (Current always
*

*> > makes a loop; when signal current traverses the via,
*

*> > a returning signal current flows SOMEWHERE in
*

*> > the opposite direction.). It is a principle
*

*> > of Maxwell's equations that high-speed returning signal
*

*> > current will flow in whatever path produces the
*

*> > least overall inductance.
*

*> >
*

*> > Let's do an example involving a signal via that
*

*> > dives down through a thick, multi-layer board.
*

*> > If the signal in question changes reference
*

*> > planes as it traverses the via,
*

*> > then the returning signal current will also have to
*

*> > change planes, meaning that the returning signal
*

*> > current will flow through one or more vias (often
*

*> > leading to bypass capacitors) as it moves from
*

*> > plane to plane. For example, if the signal starts
*

*> > out on the top layer, the returning signal current
*

*> > is flowing on the nearest reference plane (call it
*

*> > layer 2). If the via conducts the signal current
*

*> > down to the bottom layer (16), then the returning
*

*> > signal current at that point must be flowing on
*

*> > the nearest (bottom-most) reference plane, call it 15.
*

*> > Somehow the returning signal current has to hop from
*

*> > reference plane 2 to reference plane 15 in the
*

*> > vicinity of the via.
*

*> >
*

*> > If you examine the space between the planes, the
*

*> > magnetic fields within are created partly by
*

*> > the signal current, and in equal measure (but in
*

*> > differnt locations) by the returniing signal
*

*> > current, which flows on different vias. The
*

*> > total magnetic flux between the outgoing and
*

*> > returning vias defines the inductance.
*

*> > Specifically, to calculate the effective
*

*> > inductance of via (A), you must first specify the
*

*> > location of the return path, via (B), and then
*

*> > calculate the total magnetic flux in the area
*

*> > between the two vias. The total magnetic flux
*

*> > generated by a signal current of one amp, in units
*

*> > of webers, equals the inductance.
*

*> > In the case of more complex return-path
*

*> > configurations, other considerations apply.
*

*> > I think at this point that the following
*

*> > formulii for the effective series inductance
*

*> > of a via are pretty good:
*

*> >
*

*> > For a signal which pops from one side of the
*

*> > plane, through a via, to the opposite side
*

*> > of the same plane (i.e., the return current
*

*> > doesn't have to jump planes), the via
*

*> > inductance is very, very low. This is a best-case
*

*> > scenario. I don't know a good way to make this
*

*> > calculation except with a true 3-D E&M field solver.
*

*> >
*

*> > For a signal which first uses reference-plane A,
*

*> > and then changes (through a via) to use
*

*> > reference-plane B, I'll do several examples. In
*

*> > all cases the separation between reference planes
*

*> > is H. (It doesn't matter if there are other
*

*> > unused reference planes in the way, only the
*

*> > spacing between the two reference planes A and B
*

*> > matter).
*

*> >
*

*> > If the return current is carried mainly on one nearby
*

*> > via, where the spacing from signal via to return via
*

*> > is S and the via diameter is D:
*

*> >
*

*> > L = 5.08*H*(2*ln(2*S/D)) [1]
*

*> >
*

*> > If the return current is carried mainly on two vias
*

*> > equally spaced on either side of the signal via,
*

*> > where the spacing from signal via to either return via
*

*> > is S and the via diameter is D:
*

*> >
*

*> > L = 5.08*H*(1.5*ln(2*S/D) + 0.5*ln(2)) [2]
*

*> >
*

*> >
*

*> > If the return current is carried mainly on four vias
*

*> > equally spaced in a square pattern on four sides
*

*> > of the signal via, where the spacing from signal via
*

*> > to any return via is S and the via diameter is D:
*

*> >
*

*> > L = 5.08*H*(1.25*ln(2*S/D) + 0.25*ln(2)) [3]
*

*> >
*

*> > If the return current is carried mainly on a
*

*> > coaxial return path completely encircling the signal
*

*> > via, where the spacing from signal via
*

*> > to the return path is S and the via diameter is D:
*

*> >
*

*> > L = 5.08*H*(ln(2*S/D)) [4]
*

*> >
*

*> > The last formula I hope you will recognize as the
*

*> > inductance of a short section of coaxial cable with
*

*> > length H and outer diameter 2*S. I hope this
*

*> > recognition will lend credence to the idea that
*

*> > the position of the returning current path is
*

*> > an important variable in the problem.
*

*> >
*

*> > My earlier formula was a gross approximation which
*

*> > ignored the position of the returning current path,
*

*> > and omission which I greatly regret. It made the
*

*> > crude assumption that the return path was approximately
*

*> > coaxial and located at a distance S=5.43*H. As you
*

*> > note, when the inductance really matters a
*

*> > more accurate approximation is needed.
*

*> >
*

*> > To obtain a result as low as 5.08*H*(ln(2*S/D)-1)
*

*> > you would have to assume the return path were coaxial
*

*> > and located at a ridiculously small separation of
*

*> > S=.735*H, or that the return path were a single via
*

*> > located at some even closer distance.
*

*> >
*

*> > On my web site http://signalintegrity.com under "articles"
*

*> > there is a write-up about calculating the inductance of
*

*> > a bypass capacitor that includes the above formulas for
*

*> > vias, as well as some handy ways to estimate the
*

*> > inductance of the capacitor body.
*

*> >
*

*> > By the way, if you find a flaw in THIS write-up,
*

*> > please let me know.
*

*> >
*

*> > Best regards,
*

*> > Dr. Howard Johnson
*

*> >
*

*> >
*

*> >
*

*> >
*

*> >
*

*> >
*

*> > >>On the two versions of the equation, it looks to me like
*

*> the version
*

*> > >>in Johnson's book has a typo. When d>>r, the external partial
*

*> > >>self-inductance of a straight round wire is
*

*> > >>
*

*> > >>L=5.08d*{ln(2d/r)-1}nH,
*

*> > >>
*

*> > >>where d is the length in inches, and r is the radius in inches.
*

*> > >>The external inductance is a good approximation at high
*

*> frequencies
*

*> > >>where the skin effect shields the internal metal of the wire. At
*

*> > >>low frequencies, the internal self-inductance needs to be
*

*> > >>added to the external partial self-inductance to obtain
*

*> > >>
*

*> > >>L=5.08d*{ln(2d/r)-3/4}nH,
*

*> > >>
*

*> > >>which is the formula from Gover, as Eric pointed out.
*

*> > >>
*

*> > >>It seems that Johnson's book has the first
*

*> (high-frequency) version
*

*> > >>with a sign error on the 1 because he has
*

*> > >>
*

*> > >>L=5.08h*{ln(4h/d)+1}nH,
*

*> > >>
*

*> > >>where h is the length in inches, and d is the diameter in inches.
*

*> > >>
*

*> > >>
*

*> > >>This formula should not be used for vias because it assumes that
*

*> > >>the length is much greater than the diameter. To compute partial
*

*> > >>self-inductance for vias, you should use the more complex formula
*

*> > >>that does not have this assumption built in. The correct formula
*

*> > >>is (5.49) from my book. This is the external partial
*

*> self-inductance,
*

*> > >>so if you want the low frequency inductance, you need to add the
*

*> > >>internal inductance from (5.45).
*

*> > >>
*

*> > >>Finally, Grover does not actually derive much in his book. If you
*

*> > >>are interested, the round wire formula above and many others are
*

*> > >>derived in my book.
*

*> > >>
*

*> > >>Regards,
*

*> > >>Brian Young
*

*> > >>
*

*> > >>
*

*> > >>Eric Bogatin wrote:
*

*> > >>>
*

*> > >>> Itzhak-
*

*> > >>>
*

*> > >>> you asked the question about the difference in the
*

*> approximations
*

*> > >>> for the partial self inductance of a via that were given by
*

*> > >>> myself and Howard Johnson. I wanted to provide some
*

*> > >>> clarification. You wrote:
*

*> > >>>
*

*> > >>> (4) While calculating vias inductance, I've encountered
*

*> 2 similar
*

*> > >>> but
*

*> > >>> different equations for this parameter. One is given by Mr. H.
*

*> > >>> Johnson
*

*> > >>> in his famous book (page 259), as follows:
*

*> > >>>
*

*> > >>> L=5d*{ln(2d/r)+1}nH.
*

*> > >>>
*

*> > >>> The other is given by Mr. Bogatin in one of his
*

*> articles, and is:
*

*> > >>>
*

*> > >>> L=5d*{ln(2d/r)-3/4}nH.
*

*> > >>>
*

*> > >>> Can somwone explain the reason for the difference, or who is
*

*> > >>> right? The
*

*> > >>> difference starts to be quite critical when dealing with u-Vias!
*

*> > >>>
*

*> > >>> The approximation is for the partial self inductance of a round,
*

*> > >>> solid rod, of radius, r and length d. The length is in units of
*

*> > >>> inches, while the inductance is in units of nH.
*

*> > >>>
*

*> > >>> This is the approximation that was originally derived by Fred
*

*> > >>> Grover, in his classic book, Inductance Calculations",
*

*> in 1946. I
*

*> > >>> just re-checked the one I offered, and it is correctly
*

*> reproduced
*

*> > >>> above. It is listed on page 35, eq 7, of his book. I
*

*> think it has
*

*> > >>> since been reprinted as a Dover Book.
*

*> > >>>
*

*> > >>> Keep in mind two things when using this approximation:
*

*> 1st, it is
*

*> > >>> an approximation. Grover says it is good to about 2%. I have
*

*> > >>> found good agreement to better than 5% for wire bond structures.
*

*> > >>> Approximations are wonderful tools to assist you in exploring
*

*> > >>> design space, run in a spread sheet and play what-if trade offs.
*

*> > >>> They give you good answers and let you see the geometry and
*

*> > >>> materials trade offs. However, they are APPROXIMATIONS. You
*

*> > >>> should never use an approximation in a situation where the
*

*> > >>> accuracy of the answer may cost you significant time
*

*> and expense.
*

*> > >>> You should be using a 3D field solver that you have confidence
*

*> > >>> in. One of the second order effects in this approximation, for
*

*> > >>> example, is that it includes the "internal" self inductance. As
*

*> > >>> the skin depth gets to be comparable to the geometrical cross
*

*> > >>> section, the partial self inductance will decrease and reach a
*

*> > >>> constant value when all the current is in the outer surface.
*

*> > >>>
*

*> > >>> The second thing to keep in mind when using this
*

*> approximation is
*

*> > >>> that it is for the PARTIAL self inductance of the via, under the
*

*> > >>> assumptions of uniform current flow down the long axis. If you
*

*> > >>> are using it in a situation where the length of the structure is
*

*> > >>> comparable to the diameter, ie, d ~ 2r, the current distribution
*

*> > >>> through the structure may not be even close to parallel to the
*

*> > >>> long axis. Further, the actual loop inductance, which is what
*

*> > >>> matters in a real circuit, is probably dominated by other
*

*> > >>> elements than this small, squat element. The partial self
*

*> > >>> inductance may depend strongly on the proximity of other
*

*> > >>> conductors and how it affects the current flow through this via.
*

*> > >>> If you are in a regime where worrying about the presence of the
*

*> > >>> -3/4 term is important, you probably want to use a 3D field
*

*> > >>> solver before any design signoff. A good 3D solver will
*

*> calculate
*

*> > >>> the actual current distribution through the via
*

*> structure and the
*

*> > >>> rest of the current path.
*

*> > >>>
*

*> > >>> I hope this helps.
*

*> > >>>
*

*> > >>> If anyone is interested, I have various application
*

*> notes related
*

*> > >>> to approximations to inductance and general principles
*

*> related to
*

*> > >>> inductance posted on our web page. These are listed as app notes
*

*> > >>> with index numbers: 33, 32, 29, 25, and 9. You can find them
*

*> > >>> under application notes at www.gigatest.com
*

*> > >>>
*

*> > >>> As always, comments are welcome.
*

*> > >>>
*

*> > >>> --eric
*

*> > >>>
*

*> > >>> From: Itzhak Hirshtal [mailto:[email protected]]
*

*> > >>> Sent: Monday, March 12, 2001 09:33
*

*> > >>> To: si-list
*

*> > >>> Subject: [SI-LIST] : Inductance and Decoupling
*

*> > >>>
*

*> > >>> Hello, all
*

*> > >>>
*

*> > >>> I've recently started to calculate the de-coupling needed for
*

*> > >>> efficiently supplying the spike currents needed by high-speed
*

*> > >>> devices.
*

*> > >>> During this task, I've encountered several ambiguities and
*

*> > >>> results that
*

*> > >>> I would like to share with you and perhaps hear some (useful)
*

*> > >>> feedback
*

*> > >>> from you.
*

*> > >>>
*

*> > >>> (1) I tried to evaluate the situation for one high-pin-count
*

*> > >>> device with
*

*> > >>> several buses connected to it (essentially a bus bridge). Even
*

*> > >>> calculating for just one synchronous bus (with 144 bits overall)
*

*> > >>> I
*

*> > >>> arrived to the result that a few Amps (maybe even 5) are drawn
*

*> > >>> when all
*

*> > >>> or most of this bus bits change state. I wonder what will be the
*

*> > >>> result
*

*> > >>> if I would calculate for an additional bus (assuming it's
*

*> > >>> synchronous
*

*> > >>> with the first). And what about the internal changes? They might
*

*> > >>> be
*

*> > >>> contributing even more than the external bus! (e.g.,
*

*> the Motorola
*

*> > >>> PowerPC HW manual states that 90% of the power consumption of
*

*> > >>> this
*

*> > >>> device is drawn internally, not externally).
*

*> > >>>
*

*> > >>> (2) I've also tried to calculate the inductance of the
*

*> decoupling
*

*> > >>> capacitors connections to the device. Even assuming a
*

*> 40-mil wide
*

*> > >>> 50-mil
*

*> > >>> long trace right above a reference plane for the connection I
*

*> > >>> have app.
*

*> > >>> L=150-200pH. If I can't connect at least one of the capacitor
*

*> > >>> pads so
*

*> > >>> short I might have to do a direct connection via to a reference
*

*> > >>> plane. I
*

*> > >>> calculated this to have more than L=1nH!
*

*> > >>>
*

*> > >>> (3) I assumed the calculated peak currents change at a rate
*

*> > >>> equivalent
*

*> > >>> to the rise time of the device's output buffers. I don't know if
*

*> > >>> it's
*

*> > >>> true, but this seems to me the most logical thing to do. Even if
*

*> > >>> I take
*

*> > >>> it to be 2ns (1 ns is closer to worst-case, I believe), I get
*

*> > >>> the
*

*> > >>> result that I need 40 to 50 low-ESL decoupling
*

*> capacitors for the
*

*> > >>> case
*

*> > >>> where L=1nH. Only if I succeed to connect the
*

*> capacitors directly
*

*> > >>> and
*

*> > >>> close enough to both GND and VDD pins (L=150-200pH) do I get the
*

*> > >>> result
*

*> > >>> that it is sufficient to use 4-6 decoupling capacitors.
*

*> > >>>
*

*> > >>> (4) While calculating vias inductance, I've encountered
*

*> 2 similar
*

*> > >>> but
*

*> > >>> different equations for this parameter. One is given by Mr. H.
*

*> > >>> Johnson
*

*> > >>> in his famous book (page 259), as follows:
*

*> > >>>
*

*> > >>> L=5d*{ln(2d/r)+1}nH.
*

*> > >>>
*

*> > >>> The other is given by Mr. Bogatin in one of his
*

*> articles, and is:
*

*> > >>>
*

*> > >>> L=5d*{ln(2d/r)-3/4}nH.
*

*> > >>>
*

*> > >>> Can somwone explain the reason for the difference, or who is
*

*> > >>> right? The
*

*> > >>> difference starts to be quite critical when dealing with u-Vias!
*

*> > >>>
*

*> > >>> Thanks for anyone who makes the effort to read this email.
*

*> > >>
*

*> > >>
*

*> > >>--
*

*> > >>***************************************************************
*

*> > >>* Brian Young phone: (512) 996-6099 *
*

*> > >>* Somerset Design Center fax: (512) 996-7434 *
*

*> > >>* Motorola, Austin, TX [email protected] *
*

*> > >>***************************************************************
*

*> > >>
*

*> > >>**** To unsubscribe from si-list or si-list-digest: send e-mail to
*

*> > >>[email protected] In the BODY of message put:
*

*> UNSUBSCRIBE
*

*> > >>si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
*

*> > >>si-list archives are accessible at http://www.qsl.net/wb6tpu
*

*> > >>****
*

*> > >>
*

*> > >>
*

*> > >_________________________________________________
*

*> > >Dr. Howard Johnson
*

*> > >tel 425.556.0800 fax 425.881.6149
*

*> > >Signal Consulting, Inc.
*

*> > >16541 Redmond Way #264
*

*> > >Redmond, WA 98052
*

*> > >http://signalintegrity.com -- High-Speed Digital Design
*

*> > >books, tools, and workshops
*

*> > >
*

*> >
*

*> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
*

*> > [email protected] In the BODY of message put: UNSUBSCRIBE
*

*> > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
*

*> > si-list archives are accessible at http://www.qsl.net/wb6tpu
*

*> > ****
*

*> >
*

*>
*

*>
*

*> **** To unsubscribe from si-list or si-list-digest: send e-mail to
*

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*>
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*>
*

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**Next message:**Sainath Nimmagadda: "Re: [SI-LIST] : Re: approximations for partial self inductance"**Previous message:**Jian X. Zheng: "RE: [SI-LIST] : Re: approximations for partial self inductance"**Maybe in reply to:**Sainath Nimmagadda: "[SI-LIST] : Re: approximations for partial self inductance"**Next in thread:**Reid, Chris: "RE: [SI-LIST] : Re: approximations for partial self inductance"

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