These are very good points to which I would add another: current needs to be supplied to a device at its switching frequency. If you have placed a ferrite with significant impedance between the device and the power planes, then all the current is taken from local capacitors. The implication of such an arrangement is that the device generates noise which you are trying to localize and keep from the power planes. However, as current is taken from the local cap, it needs to be replaced from the plane by current flowing through the ferrite. Since there is significant impedance at this frequency, the bucket drains faster than it can be refilled. As a result you can't use a ferrite with significant impedance at the switching frequency. In general ferrites are selected to be useful at high harmonic frequencies. Curt Curt McNamara. P.E.=20 Senior Electrical Engineer=20 Logic Product Development=20 411 Washington Ave. N Suite 101=20 Minneapolis, MN 55401=20 Tel 612-436-5178=20 Fax 612-672-0443=20 curtm@xxxxxxxxxxx=20 www.logicpd.com=20 This message (including any attachments) contains confidential information intended for a specific individual and purpose, and is protected by law. If you are not the intended recipient, you should delete this message and are hereby notified that any disclosure, copying, or distribution of this message, or the taking of any action based on it, is strictly prohibited. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Zhangkun Sent: Sunday, April 09, 2006 11:00 PM To: joel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Ferrite bead question Dear Joel As to my experience, it is not enough to select bead based on the P/N. The decoupling circuit should be considered very carefully. In several cases, we use beads and the noise become larger. The following element should be considered: 1.The resonance between bead and capacitor. 2.The frequency span of the noise source. A simulation is suggested. It is very simple. Hope this is helpful Best Regards Zhangkun 2006.4.10 ----- Original Message -----=20 From: "Joel Brown" <joel@xxxxxxxxxx> To: <si-list@xxxxxxxxxxxxx> Sent: Saturday, April 08, 2006 2:33 AM Subject: [SI-LIST] Ferrite bead question > I often see recommendations in application notes for ferrite beads on=20 > DC > power lines. Sometimes a particular P/N is specified and sometimes just=20 > an impedance is given. > Wouldn't I want to choose a ferrite bead that has the highest impedance=20 > possible at the frequencies of interest, and that the current rating and=20 > DC resistance are appropriate? Would the frequencies of interest be the=20 > fundamental switching frequency of the part and possibly the 3rd and 5th=20 > harmonics? > Thanks - Joel >=20 >=20 >=20 > -- Binary/unsupported file stripped by Ecartis -- > -- Type: text/x-vcard > -- File: joel.vcf >=20 >=20 > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >=20 > or to administer your membership from a web page, go to:=20 > //www.freelists.org/webpage/si-list >=20 > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >=20 > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ >=20 > List technical documents are available at: > http://www.si-list.org >=20 > List archives are viewable at: =20 > //www.freelists.org/archives/si-list > or at our remote archives:=20 > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 >=20 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //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 =20 ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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