Hi Steve, You are right. I should use available or effective capacitance in my statement. I definitely agree that the board stackup is critical in high speed design. Thanks. Actually this is also the reason behind my suggestion #3. At high frequency(>400Mhz), the local decoupling capacitor will not be effective anymore. The build-in capacitance between PWR and GND provides most transite current at high frequency, so it is better that the vias directly connect PWR/GND pins to PWR/GND planes. Thanks for your suggestion. Zhiping Zhiping "S. Weir" wrote: > Zhiping, I would rephrase your statement to read "the goal is to minimize > the total loop inductance between the IC and the available > capacitance". Assuming power / gnd planes, the via and pad pattern of the > capacitor together with which surface it is on, and the board stackup > become dominant contributors to the inductance. > > Regards, > > Steve. > At 10:46 AM 7/19/01 -0700, you wrote: > >Hi, > > > >Here is my $0.02 input. > > > >Where to put the decoupling caps is a complicated problem. > >The PCB stackup is a big factor to consider. No matter where > >you put it, the goal is to minimize the total loop inductance > >between IC and caps. > > > >Here is what I would recommend: > >1. If you can place IC and caps pretty near each other, then > >placing them on the same side is better than the opposite side. > >2. If IC and caps are on the same side and the power layer is near IC, > >then it's better to place cap near GND pin. If the ground layer is > >near IC, then it is better to place cap near power pin. Here is a plot > >of suggested placement > > > > - IC - -cap- > > | | | | > >--*----------*------- GND > > | | > >------*--*-----------PWR > > > >3. I would recommend to connect IC and cap PWR/GND pins directly > >to PWR/GND layer through vias. > > > >Thanks. > > > > > > > >johnni.friis@xxxxxxxxx wrote: > > > > > Hi folks > > > > > > Where do you place the caps ? > > > The caps and the IC is on the side of the PCB, or ? > > > > > > 1. Close to Vcc pin > > > The GND connection of the IC and the cap are made directly to GND-plane= > > > . > > > The Vcc connection is made from Vcc pin to cap to via plane. (See below= > > > ). > > > > > > ___ > > > Vcc pin o-----| |----o via to Vcc plane > > > |cap| > > > |___|-----o via to Gnd plane > > > > > > 2. Close to GND pin > > > The Vcc connection of the IC and the cap are made directly to Vcc-plane= > > > . > > > The GND connection is made from GND pin to cap to via to plane. (See > > > below). > > > > > > ___ > > > Gnd pin o-----| |----o via to Gnd plane > > > |cap| > > > |___|-----o via to Vcc plane > > > > > > Some say 2 is best. The current from cap to IC will generate a noise > > > voltage in the plane (1. in GND-plane and 2. in Vcc-plane). > > > It's best to have the noise in Vcc-plane. > > > (Our baords is 4 layers, plane seperation of 1 mm (40 mill), F=3D40 M= > > > Hz, > > > singel ended signals, tr/tf ~ 1nS. ) > > > What do you think ? > > > > > > Best regards, > > > > > > Johnni Friis > > > G=E5sdal Bygningsindustri A/S > > > Decoration & Sunscreening, Electrics and Shutters > > > B=E6kg=E5rdsvej 40 > > > DK 6900 Skjern > > > E-mail: johnni.friis@xxxxxxxxx > > > Phone +45 9980 6666 > > > Fax +45 9980 6260= > > > > > > ------------------------------------------------------------------ > > > 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 > > > > > > >-- > > Zhiping Yang, Ph. D. > > Hardware Engineer > > Cisco Systems > > 270 West Tasman Drive > > Mail Stop:SJCG/2/2 > > San Jose, CA 95134 | | > > email: zhiping@xxxxxxxxx :|: :|: > > Tel : 408 525 5690 :|||: :|||: > > Fax : 408 526 5504 .:|||||||:..:|||||||:. > >***************************************************** > > > > > > > > > >------------------------------------------------------------------ > >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 > > > > ------------------------------------------------------------------ > 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 > ------------------------------------------------------------------ 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