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Thread: RESPONSE: 5V Logic Bypass caps - SMD value/phy size
www.piclist.com/techref/index.htm?key=logic+bypass+caps
picon face BY : Russell McMahon email (remove spam text)



>> 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.


Practical capacitors are unfortunately not purely capacitive nature.
They also have inductive and resistive components.
Larger value capacitors generally have higher internal inductance and lead
inductance than smaller valued capacitors of the same type.
Consequently the impedance of a given capacitor will generally have a a
minimum at a certain frequency and the impedance will be higher and both
lower and higher frequencies. The rule of thumb that was mentioned above is
in the order of correct for ceramic capacitors.
ie The "correct" value with short leads for decoupling in the MHz plus
region is around 0.1uF. As the frequency rises a smaller capacitor will be
optimum.
The excessively enthused can even specify capacitors based on the series
resonant combination of lead lengths and capacitance.
ARRL handbook gives these figures for series resonance (optimum bypassing)
for disk ceramics with total lead lengths of 0.5 inch.

Cap uF     Freq MHz

0.01            15
.0047          22
.002            38
.001            55
.0005          80
.0001        165

Sounds like we should be using 100 pF decoupling with 100 MHz Scenix's ! :-)

RF practice (and serious microprocessor practice in some cases) is to group
several capacitors of different values together to combine the
characteristics of each = effectively a rather broad bandpass filter. Use of
small ceramics and larger valued distributed electrolytics (tantalum for the
brave, solid aluminium for the wise, wet electrolytic for the adventurous)
can be useful.



RM

<002201bfcf8f$88b06dc0$b84b61cb@mkbs> 7bit

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