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GARTHWILSON
PostPosted: Mon May 07, 2012 2:04 am 
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Quote:
However, the board itself contributes capacitance that is difficult to estimate.

It's not difficult to measure though. :wink: I just measured from one of the traces on the memory module that runs along a wide ground trace, to both power and ground combined, and got 4pF. I tried to pick the worst one. Next I measured a pin header and socket combination, one pin to the five pins around it combined (those other five connected together for the test) and got 2pF. The resolution on the meter is 1pF and the reading was vacillating between 1 and 2pF but spending more time at 2pF. The conclusion is that the board itself, including the in header and socket, will have very little capacitance compared to the ICs on it. Granted, it's not a multi-layer board.

Edit: Next, on this multilayer board I've shown before
Image
(which has the ROM on the other side), I measured the capacitance of the longest .007"-wide trace on a layer that's only separated from a plane layer by about .005-.006", and got 14pF. There's yet another advantage of using small SMT parts and getting them close together and keeping the board small: the shorter traces lets you can minimize the capacitance between traces and the planes. This is ridiculous though. I filed into the edge to get to pads along the edge to see the layer spacing under the microscope. I did not specify to the manufacturer how the six layers should be spaced, and they sure didn't space them evenly. From this layer to the next layer on the other side was 2-3 times as far as to the nearby plane layer.

Quote:
There are a number of variables that get into the picture [...] the dielectric constant of the board material

That's 4.1 for the standard FR-4 board material.

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BigDumbDinosaur
PostPosted: Mon May 07, 2012 5:36 pm 
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GARTHWILSON wrote:
Quote:
There are a number of variables that get into the picture [...] the dielectric constant of the board material

That's 4.1 for the standard FR-4 board material.

I couldn't recall the number. Too many numbers and not enough synapses. :D

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