5V Logic Bypass caps - SMD value/phy size recommendations?
Robert A. LaBudde email (remove spam text)
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<x-flowed>At 10:49 AM 6/2/00 -0700, Dave wrote:
> >Has anyone found/experimented with the optimum size capacitance
> >value and/or physical size for use in/around a microprocessor?
>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.
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.
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.
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.
Attachment converted: creation:capacitor impedance vs freq.gif (GIFf/JVWR) (00016199)
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: lcfltd.comral
Least Cost Formulations, Ltd. URL: http://lcfltd.com/
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Virginia Beach, VA 23464-3239 Fax: 757-467-2947
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See also: www.piclist.com/techref/index.htm?key=logic+bypass+caps
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