[SI-LIST] Re: DC resistance measurement
- 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
>
>
>
>
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