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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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At 10:35 AM 6/2/00 -0500, Jim P wrote:
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You want the caps to absorb well at the third harmonic of the clock.
0.1uF does well at 3 MHz, 0.01 at 30. 0.001 at 300.
It's a broad response, so dont think that there's one specific value.
However, if you use 0.1uF on a 20 MHz part, you won't get the supression
that you could if you used 0.047uF.


>I've got access to the under side of the board - and that's where
>I had intended on placing the SMD bypass caps.

Place the cap at the ground pin, and route power to the cap first, and from
there to the chip. Otherwise, you compromise the bypass.
Murata and panasonic make through-hole 3 leaded caps which help eliminate
this by having ground, in and out leads (I and O are reversible)

>The first few boards will be hand-built prototypes (of course!) - so
>am I better off with the larger size (1210 etc) SMD caps?

I do tons of work with 1206 and smaller with no special tools, though 0402
is pretty difficult.

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