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Old 12th February 2003, 05:01 AM   #1
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Default Capacitors for digital circuits- Tants vs electrolytics

Hi All,

What is an optimal cap for digital circuitry? I have seen suggestions to:
1. Use tantalum caps since they "ring" (not audible of course) less when in use
2. Or use very large (1000uF) Panasonic FC or FA caps for local decoupling
3. Use a combination of cpas for decoupling (ala Pete Goudreau)

There would appear to be merit in both 1 and 3, but I am not so sure about 2.

Any comments?
Thanks
Ryan
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Old 12th February 2003, 07:10 AM   #2
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Default Re: Capacitors for digital circuits- Tants vs electrolytics

[QUOTE]Originally posted by Dr.H
[B]Hi All,

What is an optimal cap for digital circuitry?
------------------------------------------------------
OSCons are quite small and have very low impedance and good frequency characteristics. I find them excellent as digital bypasses etc.

I have used tantalum extensively when there is no space or when there are cost limits, especially in power supplies. Their high distortion does not seem to matter.

Large lyics have poor high frequency characteresitics despite claims to the contrary. To me, they are just too large to force fit on SMT boards.

SMT caps are best as parallel bypasses to arger caps.
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Old 12th February 2003, 08:44 AM   #3
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Heavy thread here, but worth reading . Not very easy to navigate through the multiple curves, but in conclusion, it appears that a cheap Panasonic 100uF FC paralleled with a high value (4.7uF) SMT ceramic from Murata, plus some small SMT ceramics do a good job.

Hope this helps
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Old 12th February 2003, 08:54 AM   #4
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Quote:
Originally posted by CheffDeGaar
Heavy thread here, but worth reading . Not very easy to navigate through the multiple curves, but in conclusion, it appears that a cheap Panasonic 100uF FC paralleled with a high value (4.7uF) SMT ceramic from Murata, plus some small SMT ceramics do a good job.

Hope this helps
--------------------------------------------
Unless ones designs the PCBs from scratch, most of the time it is not possible, or very messy to do this
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Old 12th February 2003, 01:02 PM   #5
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decouple all the power supply pins with 100Nf ceramic. for ADC's I use a 4.7uF tantalum with a 100Nf ceramic. I surround the ADC with a ground plane. A decoupling cap without a ground plane is about as useful as the tits on a bull.
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Old 12th February 2003, 01:22 PM   #6
tiroth is offline tiroth  United States
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Ever tried a big (3-10uF X5R) ceramic in place of the tantalum? I would think it a lot better there. I don't have the equipment to measure it though.
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Old 12th February 2003, 03:11 PM   #7
ALW is offline ALW  United Kingdom
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Default Decoupling

I'd second the use of OS-CONs they are really good.

As a cheaper alternative the Rubycon ZA's are excellent, reaching minimum, and close to organic semiconductor cap levels, at around 100-150kHz.

You can then bypass with a suitable HF cap (ceramic / PPS) to take over from here - surface mounts are ideal for fitting across existing cap leads, on the underside of a PCB.

Andy.
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Old 12th February 2003, 03:21 PM   #8
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Here's the link to the similar mods I did on my DAC The mods never end, adding Goudreau's triplets
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