Maybe someone can explain it, but I've never understood adding _any_ additional impedance to a reference plane (call it "ground") connection. It seems I should always be aiming for the common impedance here for reference purposes. Creating an isolated power to reference current loop does seem to show advantage when both high speed digital and low level analog live on the same board. Am I missing something? Regards, Brent DeWitt Datex-Ohmeda -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Martin Euredjian Sent: Wednesday, November 20, 2002 3:31 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Decoupling of Oscillator From: Philip Ross Wellington > Some provided Pi filters for the > ground but that didn't work well logically because of the return path > inductance and Signal Integrity (not even coined back then) induced > problems. On datasheets I've seen recommended isolation/filtering networks that look like this: (values chosen at random) RAW_VCC -----------BEAD-------------- VCC' | | | | | | | | 22uF 0.1uF 0.1uF 22uF | | | | | | | | RAW_GND -----------BEAD-------------- GND' versus: RAW_VCC -----------BEAD-------------- VCC' | | | | | | | | 22uF 0.1uF 0.1uF 22uF | | | | | | | | RAW_GND ----------------------------- GND' Is there any merit to the first approach, say, for a clock, or a device with a PLL, or a sensitive analog sub-section? Both in terms of circuit operation and EMI/RFI concerns. I've also seen a couple where the beads are replaced with low or 0 ohm resistors. Thanks, =============================== Martin Euredjian eCinema Systems, Inc. voice: 661-305-9320 fax: 661-775-4876 martin@xxxxxxxxxxxxxx www.ecinemasys.com =============================== ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list 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 or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list 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