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 field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu