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[SI-LIST] Re: decoupling
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 12 Jul 2001 14:26:03 -0700
Steve,
The higher the speed of the differential outputs, the more tightly balanced they
will have to be. Even with tens of such outputs, I expect that power pins
dedicated to this section will be quieter than the digital power rail.
Thanks,
Vinu
"S. Weir" wrote:
> Vinu, that's only true if on the diff part side of the filter, the power
> rail on the chip side is quieter than the planes. That's kind of hard to
> do unless we are just talking about a discrete driver chip that is balanced
> very well. Maybe your concern is with PECL, or CML where rail noise
> between the driver and receiver can cause trouble.
>
> Regards,
>
> Steve.
> At 12:58 PM 7/12/01 -0700, you wrote:
>
> >Steve,
> >
> >Powering a device with differential outputs directly from a digital power
> >plane
> >with high frequency switching noise can cause eye closure.
> >
> >Introducing an impedance in series to block such noise will improve the
> >differential output signal quality.
> >
> >Thanks,
> >Vinu
> >
> >"S. Weir" wrote:
> >
> > > Vinu, a series impedance can block return currents, which is a good thing
> > > for analog supplies in mixed-signal parts. So we agree on the PLL
> > > front. But, I cannot think of a case where inserting an impedance is
> > > going
> > > to help the performance of a device with differential digital
> > > outputs. Have you seen such a case? If CM noise is the concern, CM
> > > magnetics are the typical, and effective solution.
> > >
> > > Regards,
> > >
> > > Steve.
> > > At 09:35 AM 7/12/01 -0700, you wrote:
> > >
> > > >I think it should be clarified here that inserting a series AC impedance
> > > >is bad
> > > >only when the power lead powers single ended digital outputs.
> > > >
> > > >If the power lead supplies circuits that draw constant current (analog
> > > >PLL
> > > >supply or differential outputs), a series AC impedance can improve
> > > >performance.
> > > >
> > > >Thanks,
> > > >Vinu
> > > >
> > > >"S. Weir" wrote:
> > > >
> > > > > Michael,
> > > > >
> > > > > The device switching currents are fed by the board power
> > system. If you
> > > > > insert an impedance, you are choking the power supply and the on-chip
> > > > > voltage rail drops. That is a very bad thing.
> > > > >
> > > > > Regards,
> > > > >
> > > > > Steve.
> > > > > At 05:27 PM 7/11/01 -0500, you wrote:
> > > > >
> > > > > >Can you explain why adding ferrite bead in series with a power
> > lead is a
> > > > > >bad thing? Thanks.
> > > > > >
> > > > > >Regards,
> > > > > >Michael Chan
> > > > > >
> > > > > >
> > > > > >-----Original Message-----
> > > > > >From: Ritchey Lee [mailto:leeritchey@xxxxxxxxxxxxx]
> > > > > >Sent: Wednesday, July 11, 2001 4:48 PM
> > > > > >To: si-list@xxxxxxxxxxxxx
> > > > > >Subject: [SI-LIST] Re: decoupling
> > > > > >
> > > > > >
> > > > > >
> > > > > >Yes, at 250 MHz and up, the plane capacitor is the work
> > > > horse. Imagine what
> > > > > >happens when someone tells you to insert a ferrite bead in series
> > with a
> > > > > >power
> > > > > >lead, isolating it from the plane capacitor. Really bad
> > things! Sadly,
> > > > > >many
> > > > > >applications notes tell you to do this, as do some of the less well
> > > > trained
> > > > > >EMI
> > > > > >people. I spend a lot of time fixing designs that have followed this
> > > > > >advice.
> > > > > >
> > > > > >Lee
> > > > > >
> > > > > >Khalid Ansari wrote:
> > > > > >
> > > > > > > Larry,
> > > > > > >
> > > > > > > What happens much above 250 MHz, do we need any more
> > > > > > > capacitors or is the power to ground plane capacitance
> > > > > > > sufficient?
> > > > > > >
> > > > > > > >470 pF 232 MHz
> > > > > > > >0.01 uF 50 MHz
> > > > > > > >0.1 uF 16 MHz
> > > > > > >
> > > > > > > >Putting low ESR capacitors in parallel is like playing with
> > fire. You
> > > > > > > >can cook with fire and heat your house, but if you are not
> > > > careful with
> > > > > > > >fire, you will get burned. I am a strong advocate of multiple
> > low ESR
> > > > > > > >capacitors in parallel because I believe we have learned how
> > to make
> > > > > > > >good, safe use of them. The optimum power distribution system
> > (fewest
> > > > > > > >components, least cost, least complexity) is obtained from
> > careful
> > > > > > > >selection and placement of capacitors with a reasonably high
> > Q (low
> > > > > >ESR).
> > > > > > > >
> > > > > > > >The trick is to create a low and flat impedance profile in the
> > > > > > > >frequency domain by using different valued capacitors in
> > parallel.
> > > > > > > >Systems behave best when chips look out and see a power
> > distribution
> > > > > > > >system that is resistive in phase (flat impedance). We like to
> > > > > > > >establish a target impedance which is defined as
> > > > > > > >
> > > > > > > >
> > > > > > > >Ztarget = power_supply_voltage * 5% / transient_current.
> > > > > > > >
> > > > > > > >If your PDS impedance meets the target impedance up to the
> > highest
> > > > > > > >frequency of interest, your noise will be within acceptable
> > limits.
> > > > > > > >Much more is written on this topic in
> > > > > > > >
> > > > > > > >"Power Distribution System Design Methodology and Capacitor
> > > > > > > >Selection for Modern CMOS Technology"
> > > > > > > >
> > > > > > > >http://www.qsl.net/wb6tpu/si_documents/docs.html
> > > > > > > >
> > > > > > > >It is easy to meet a 1 Ohm or 0.1 Ohm target impedance using
> > > > "rules of
> > > > > > > >thumb" for decoupling and high ESR capacitors. It becomes a
> > little
> > > > > > > >more difficult to meet a 10 mOhm target. If you are trying to
> > meet 1
> > > > > > > >mOhm target impedance up to several hundred MHz, it will be very
> > > > > > > >difficult unless you have a well defined methodology and some
> > > > software
> > > > > > > >tools to help you. On some of our more recent products, at
> > least one
> > > > > > > >of each of the ceramic capacitors from the following menu are
> > used:
> > > > > > > >
> > > > > > > >100uF,
> > > > > > > >47uF, 22uF, 10uF,
> > > > > > > >4.7uF, 2.2uF, 1uF,
> > > > > > > >470nF, 220nF, 100nF,
> > > > > > > >47nF, 22nF, 10nF,
> > > > > > > >4.7nF, 2.2nF, 1nF,
> > > > > > > >680pF, 470pF, 330pF, 220pF, 150pF, 100pF
> > > > > > > >
> > > > > > > >With three capacitors per decade of capacitance, it is possible
> > > > to make
> > > > > > > >a flat impedance vs frequency profile from about 200 kHz to
> > 400 MHz
> > > > > > > >without any problem from parallel antiresonances. The lower
> > the ESL
> > > > > > > >and ESR (within reason), the fewer components you need. X7R
> > > > capacitors
> > > > > > > >tend to have Q's between 2 and 5 and three values per decade are
> > > > > > > >sufficient. NPO (COG) capacitors may have Q's between 5 and 10
> > > > and six
> > > > > > > >values per decade are useful. Closely spaced power planes may
> > be used
> > > > > > > >instead of some of the pF capacitors.
> > > > > > > >
> > > > > > > >We have our own internal software tools to help manage the
> > design.
> > > > > > > >Cadence is marketing the Power Delivery Tool under
> > SpectraQuest that
> > > > > > > >does the same thing as our tools. The Cadence tool is even
> > > > > > > better
> > > > > > > >because it is hooked up to the design data base for the PCB.
> > > > > > > >
> > > > > > > >Like all SI tools, these tools are based on models and the
> > > > analysis is
> > > > > > > >only as good as the models. These days, I spend half of my life
> > > > out in
> > > > > > > >the lab measuring capacitors and reducing the measured data. The
> > > > > > > >capacitor vendors could help me greatly by measuring the ESR and
> > > > ESL of
> > > > > > > >their capacitors and publishing the data. They could also help
> > > > > > > by
> > > > > > > >designing capacitors that have the absolute minimum internal
> > > > > > > >inductance. BTW, traditional measurement techniques do not
> > > > > > > obtain
> > > > very
> > > > > > > >good values for ESR and ESL. (Maybe that should be the topic of
> > > > > > > >another email.)
> > > > > > > >
> > > > > > > >regards,
> > > > > > > >Larry Smith
> > > > > > > >Sun Microsystems
> > > > > > >
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