Re: Re: Bypass thoughts

nop head wrote:
> Caps have a well know V shaped self resonant curve. This is series 
> resonance from the capacitance plus lead inductance in series. When you 
> put a big cap and a small cap in parallel you might expect a W shaped 
> curve from the two Vs. In fact what I am told happens is that you get a 
> parallel resonance in the middle from the big cap that has gone mainly 
> inductive and the small cap that is still capacitive. This can make 
> things much worse as the impedance at this point is higher than either 
> of the caps would be on their own.
> 

Yes, that can certainly happen.  You can also sometimes create resonant 
loops between different cap sizes, giving you waves that oscillate 
between them.  So if you want to use multiple cap sizes to cover a 
spread of frequencies and avoid particular resonant frequencies, you 
have to use many steps - you want at least 100 nF, 10 nF and 1 nF, 
rather than just two steps.  Note that you don't get such effects 
between two caps of the same size, nor (much) between caps of 
significantly different sizes (such as bypass caps at 100 nF, and bulk 
caps at 10 uF and above).

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