Wei, essentially, you are looking at the effective loop inductance from the capacitor towards the component pins (VCC/VSS). so, the charge flow is from the VCC pin, towards the capacitor +ve terminal, and coming out of the -ve terminal, back towards the VSS pins. depending on the plane separation thickness of VCC/VSS (and that they are on the top few layers rather than bottom few layers) + how close it is to the pins, config 1 could be better than config 2. you can do the estimate either through some equation/hand calculation which will give you first order estimate, or through modeling (parasitic extraction). chee-yee -----Original Message----- From: Wei_Chen@xxxxxxxx [mailto:Wei_Chen@xxxxxxxx] Sent: Thursday, August 16, 2001 5:14 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Decoupling capacitor placement There are two ways to place a decoupling capacitor for a Vcc pin: 1. Place the cap on the top-side of the board and close together with the Vcc pin. Connect Vcc pin to the cap and use two vias from the cap to the power plane and ground plane. This is pin-cap-via topology. 2. Place the cap on the bottom side of the board (under the IC) and close to the Vcc pin. Connect Vcc to power plane through a via and connect the cap to power and ground plane through 2 vias. This is pin-via/cap-via topology. Basically you used the power plane to connect your Vcc pin and decoupling capacitor. This is a multi-power/ground plane board. Which topology would you recommend? Thanks, Wei ------------------------------------------------------------------ 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