[SI-LIST] Re: anlog and digital power plane isolation with ferrite bead good idea?

  • From: Alexandre Desnoyers <alex@xxxxxxxx>
  • To: steve weir <weirsi@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Thu, 06 Jan 2011 11:05:12 +0100

 Hello Steve,
I've been following this thread with much interest.  

I just have a problem with doing my homework:  it's almost impossible when
ICvendors do not want to provide a minimum of information.  I requested some
very specific information from the two biggest "PC compatible processor" IC
vendors, and both of them answered to use the reference design ferrite
filters ONLY, and they didn't provide the information I requested to
actuallydo my homework.

Maybe you have some tips to share with us, apart from making a test board or
changing vendor.

I had a private discussion with Istvan Novak about PLL supply noise
sensitivity characterization, and he was telling me that it was not trivial
for Sun/Oracle ASIC staff to simulate.  He also tried himself to
characterizeone IC by measurement, and found it very difficult.

I would like to suggest that the "brains" of our industry get together and
publish a test scheme that would provide us the required information.   I'm
sure that some IC vendors have such test in-house, but I haven't seen any
publications (IEEE or other) describing such test setup in details.

With the proliferation of high-speed serial link, it seems that the IC
vendors recommend one bead filter for each interface. So for a typical
processor with SATA, Ethernet, PCIe and USB3, each of these interface
"requires" an independent filter.  I would like to see a SPICE simulation
showing the interaction between each of these supplies with all the
parasiticelements, maybe then I would be convinced to use one filter for

Looking forward to hear your presentation at DesignCon.  Hopefully there
willbe representative from the major IC manufacturers in the room....


Alexandre Desnoyers
Electronic Design Engineer
Qtechnology A/S
Valby Langgade 142, 1.sal - 2500 Valby - Denmark - www.qtec.com[1]

P.S.  If I had followed the recommended filtering, my 3"x4" board would have
had 19 ferrite-bead filters for only 3 ICs.....

steve weir wrote: Dong, the key is that either someone does the homework or
there can be no guarantee that the design will work over the expected
operating conditions. Steve. Dong Kim wrote: Thanks everyone for the answers
and opinions. >From reading all the responses, I see two distinct camps of
opinions. Of course, if I get all the right parameters, files and the tools
to simulate, I will have better psudo-facts to present. Most of my recent
board designs, I did not have split power planes between analog and digital
and found no problem. May be because the board sizes are large (22"x19.5")
and 26 layers+ which includes multiple power and ground planer caps in the
stackup. What I am designing is a half size add-on card with FPGA with
multiple serdes, DDR2 memories, and other memories. I guess I will let my
PDNguy make the decision to use the ferrite since I do not see any clear
answer. I will post the result when the board is fabed and all tested out.
Happy New Year, Dong KIm, --- On Tue, 1/4/11, steve weir
<weirsi@xxxxxxxxxx>[2]wrote: From: steve weir <weirsi@xxxxxxxxxx>[3]
Subject:[SI-LIST] Re: anlog and digital power plane isolation with ferrite
bead good idea? To: "Stefan Milnor" <stefan.milnor@xxxxxxxxxxxxxx>[4] Cc:
"Cheng, Chris" <chris.cheng@xxxxxx>[5], "Tom Dagostino"
<tom@xxxxxxxxxxxxx>[6], "Joel Brown" <joel@xxxxxxxxxx>[7], "Dan Smith"
<Dan.Smith@xxxxxxxxx>[8], "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>[9], "Dong
Kim" <kimdongsik_us@xxxxxxxxx>[10], si-list@xxxxxxxxxxxxx[11], "Istvan
Novak"<istvan.novak@xxxxxxx>[12], "liuluping 41830"
<liuluping@xxxxxxxxxx>[13]Date: Tuesday, January 4, 2011, 6:59 PM Stefan
theyare narrow etch segments that form small inductors. Steve. Stefan Milnor
wrote: What is a "trench and draw bridge" - in circuit design ?
-----OriginalMessage----- From: si-list-bounce@xxxxxxxxxxxxx[14]
[mailto:si-list-bounce@xxxxxxxxxxxxx[15]] On Behalf Of Cheng, Chris Sent:
Tuesday, January 04, 2011 4:09 PM To: Tom Dagostino; 'Joel Brown'; 'Dan
Smith'; 'steve weir' Cc: 'Lee Ritchey'; 'Dong Kim';
si-list@xxxxxxxxxxxxx[16]; 'Istvan Novak'; 'liuluping 41830' Subject:
[SI-LIST] Re: anlog and digital power plane isolation with ferrite bead good
idea? First off I have to apologize for tangenting off from the original
discussion which is isolating digital and analog power planes with beads.
Personally I don't believe in them for most of the situations and at most
something like a trench and a draw bridge will probably work. But too many
discussions about applying beads in PLL filtering followed and I think we
need to clarify a few points. There always two opposite sides of PLL loop
bandwidth trade off, jitter transfer vs. jitter accumulation. If you come
from the current SerDes designers, you will most likely believe in jitter
transfer and cut your loop bandwidth to as low as you can do. If you come
from the classic PLL applications such as CPU or ASIC internal clocks, you
will most likely be more concern about core noise and jitter accumulation. I
happened to come from the later school so a natural thing to do is to push
the loop bandwidth of the PLL to as high as possible without sacrificing
phase margin too much. The beauty of it is the PLL then can self correct its
jitter up to near the PLL loop bandwidth. The side effect of this is you can
easily build an internal regulator with modest damping and pretty much clean
up the jitter from low to all the way to near the loop bandwidth frequency.
This is when the external power filtering become interesting. Because the
frequency needing protection is relatively high, the ferrite bead is ideal
for the application because it has high loss at those high frequency. That
would also allows a relatively small capacitor to form the filter tree and
itmakes a great compact external filter. The ESR of the bead can be spec to
relatively low to avoid the DC drop but I found sometimes I need a small
series resistor just to keep the Q low enough. That was the original design
when we first investigate this PLL jitter accumulation effects in the late
80's and published the app notes for our then ASIC vendors. If you come from
the "fear everything from the source jitter transfer so let's drop bandwidth
as low as we can" camp, well, then why are you worry about jitter
accumulation and build the power filter trees in the first place ? That's
like breaking your right leg and then complain about why can't you run......
But that's a debate we have to wait for the next time. Happy New Year, Chris
Cheng Distinguished Technologist , Electrical Hewlett-Packard Company +1 510
413 5977 / Tel chris.cheng@xxxxxx[17] / Email 4209 Technology Dr Fremont, CA
94538 USA -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx[18]
[mailto:si-list-bounce@xxxxxxxxxxxxx[19]] On Behalf Of Tom Dagostino Sent:
Monday, January 03, 2011 10:11 PM To: 'Joel Brown'; 'Dan Smith'; 'steve
weir'Cc: 'Lee Ritchey'; 'Dong Kim'; si-list@xxxxxxxxxxxxx[20]; 'Istvan
Novak'; 'liuluping 41830' Subject: [SI-LIST] Re: anlog and digital power
plane isolation with ferrite bead good idea? I find the "follow the
referencedesign" approach very iffy at best. Any filter that is places
between the "outside world" and the PLL or whatever only reduces the noise
from the outside world by XdB(f). So if the noise the IC vendor had in the
reference design was only 50 mV and your outside world is at 100 mV or has a
different frequency content you may have an issue. You need specs for what
the chip can tolerate, not a cook book design. Tom Dagostino Teraspeed Labs
13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@xxxxxxxxxxxxx[21]
www.teraspeed.com[22] Teraspeed Consulting Group LLC 121 North River Drive
Narragansett, RI 02882 401-284-1827 www.teraspeed.com[23] -----Original
Message----- From: si-list-bounce@xxxxxxxxxxxxx[24]
[mailto:si-list-bounce@xxxxxxxxxxxxx[25]] On Behalf Of Joel Brown Sent:
Monday, January 03, 2011 7:09 PM To: 'Dan Smith'; 'steve weir' Cc: 'Lee
Ritchey'; 'Dong Kim'; si-list@xxxxxxxxxxxxx[26]; 'Istvan Novak'; 'liuluping
41830' Subject: [SI-LIST] Re: anlog and digital power plane isolation with
ferrite bead good idea? I encounter this very frequently when it comes to
designing with parts that have separate analog and/or PLL power rails. It is
very common for manufacturers of these parts to recommend either in a
datasheet, application note or reference design to use a ferrite bead which
in theory could reduce the noise that is present on the digital power
supplies. Sometimes a manufacturers P/N is given for the ferrite bead and
sometimes no information is given. For me when dealing with a vendor a
typical scenario would be like this: Submit a question to mysupport.com
"whatare the noise and ripple requirement of the analog and PLL power
inputs"Answer one week later "Just follow the reference design, we have
tested it and it works. If you don't follow it then good luck". My options
are as follows: Do a PDN analysis of the whole board and determine if the
noise is low enough to directly connect the analog and/or PLL to the digital
power. In reality this is a guesstimate because you can with some serious
work determine with some degree of accuracy the PDN impedance but no IC
manufacturer will tell you the input power current vs frequency
characteristics of their part which is what you need to know what the noise
voltage will actually be. Do an analysis of the ferrite / capacitor network
to see how it behaves and look for problems like resonances. Replace or
supplement the ferrite bead with a linear regulator to further reduce the
noise. This only works for certain frequencies that the regulator will
rejectinput noise. In the end whatever I choose to do, I think making noise
measurements on the actual circuit is probably the most useful piece of
information. If I didn't get it right then I can tweak the ferrite or
capacitors to get it working. This hasn't happened yet. Its too bad that the
manufacturers of ICs can't come up with some standardized way specifying
current and noise on power pins. Everybody has IBIS or SPICE models of the
signal I/O pins but when it comes to power it's a black hole. Joel
-----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx[27]
[mailto:si-list-bounce@xxxxxxxxxxxxx[28]] On Behalf Of Dan Smith Sent:
Monday, January 03, 2011 1:34 PM To: steve weir Cc: Lee Ritchey; Dong Kim;
si-list@xxxxxxxxxxxxx[29]; Istvan Novak; liuluping 41830 Subject: [SI-LIST]
Re: anlog and digital power plane isolation with ferrite bead good idea?
Steve that is a good point. Dong, in my experience the most difficult piece
of the design (of late, all my designs) is getting the requirements out of
the vendors. I use a common spreadsheet that I customize for each chip
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