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Thread: 5V Logic Bypass caps - SMD value/phy size recommendations?
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face BY : David VanHorn email (remove spam text)



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>This suggests that lower capacitance a priori works better at higher speeds.
>
>My understanding of the specifications suggests the opposite: for any make
>of capacitor, higher capacitance always works better at any frequency.

That would be true if they didn't have internal inductance that is
proportional to the capacitance.


>What might be true is that certain types of capacitors are only available
>in small denominations, and these types have lower ESR and ESI so they work
>better at higher frequencies.

Smaller values of C have smaller internal inductance, in any given package.

>What's important is that the impedance of the capacitor be only 1 ohm or so
>at the frequency of interest to reduce ripple at the current draw required.

More properly, as low as possible.
A cap with a Z of 1 ohm won't do a lot in a circuit with comparable impedance.

My measurements were made in a 50 ohm system, (stripline on G-10 PCB)
Thinner tracks will be higher impedance, and a given cap will perform
better with higher track impedances. (provided of course proper layout!)




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