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 > > > > > > > > > > > > > > ------------------------------------------------------------------ > > > > > > > To unsubscribe from si-list: > > > > > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject > > field > > > > > > > For help: > > > > > > > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > > > > > > > > > > > > List archives are viewable at: > > > > > >//www.freelists.org/archives/si-list > > > > > > > Old 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 > > > > > >For help: > > > > > >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject 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