gEDA-user: bypass caps

[John Doty]

On Aug 20, 2006, at 10:56 PM, DJ Delorie wrote:

>
> 0.1 uF or 1 uF ?  Same footprint (0603), the 1uFs are a few cents more
> each (er, ~ 40% more cost for 10x the capacitance).
>
> Don't know if the 0.1uFs have some benefits inductance-wise or
> ESR-wise, in general.

If they are both 0603, inductance should be about the same. For a  
component (like a resistor or capacitor) that doesn't concentrate a  
magnetic field internally, the inductance you measure depends more on  
the test setup (particularly where the return current flows) than on  
the component. A metal slug in place of the component will give you  
about the same result. As a rough estimate, figure ~1nF/mm of  
component length assuming the return path isn't too far away (this is  
why a ground plane is good).

ESR is a whole other can of worms. It's not sensitive to test  
geometry like inductance, but it's very sensitive to the frequency  
and amplitude of the test signal.

John Doty              Noqsi Aerospace, Ltd.
jpd at wispertel.net