*From*: Istvan Novak <istvan.novak@xxxxxxxxxxx>*To*: zhang_kun@xxxxxxxxxx*Date*: Tue, 22 Aug 2006 08:38:54 -0400

Dear Zhangkun, DC measurements of PDNs can sometimes be almost as tricky as high-frequency measurements. When it comes to the DC resistance of the planes, first you have to decide what you want to measure. If we want to measure the sheet resistance of the planes, we need to keep in mind that the definition of sheet resistance assumes a uniform square (or rectangular, if we do scaling) sheet of material with UNIFORM current entering and exiting two opposite sides. So unless the rest of the plane is taken into account by correction factors, the current entry and exit must be along the full opposite edges of the planes. On a bare laminate we can approximate this condition very well with some work, but on production boards we seldom have the means to make the connections, which satisfy these conditions. If we want to measure the DC resistance between actual connection points on a real PCB, we need to remember that the connection geometry may make a big difference. With only one entry and one exit connection, the resistance approaches infinite as the size of connections approaches zero. In practical applications we may have multiple entry and exit points: high-current connections to DC-DC converters and other power sources (should) have multiple vias to the planes, and high-current chips (should) have multiple vias/pins connecting to the package. Under these circumstances it is better to measure and plot the potential surface at any and all points that you can measure. If you have a stable DC current source with high internal resistance, the source current can be measured separately through a shunt. Once you know the source current, it is connected to the DUT and the voltage drops at various points can be measured with an accurate DC voltmeter. Regards, Istvan Novak SUN Microsystems Zhangkun wrote: >Dear all >Now, we are measure DC resistance of power plane in PCB. The presision should >be 0.1 mohm at least. However, because of the clamp of the meter, the error is >about 1m ohm. In another word, the difference between two measurement for the >same DUT is about 1 mohm. Is there any advice about this topic? Anything will >be help. In your measurement, how to connect the probe to the plane? > >Best Regards > >Zhangkun >2006.8.22 > > > > ------------------------------------------------------------------ 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: http://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: http://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

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