Re: [SI-LIST] : Medium range capacitors

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From: Shayle Hirschman (shayle@mho.net)
Date: Tue Mar 07 2000 - 12:44:27 PST


Thanks, Steve, for your answers.

I have used 0.1 uF successfully in speeds to 66 MHz with fast edge rate
logic (terminated when needed) and I suspect smaller values won't really
help with higher frequencies unless a smaller, or less inductive, package
is used instead of the 0805 of the 0.1 uF caps.

I think you hit upon the crux of my client's concern, which is the voltage
regulator's output impedence at the mid range frequencies of the RS-232
transmissions. You mention that the bulk capacitor at the entry point of
the power input would help with this. I think my client would then ask
about that cap's influence elsewhere in the board, which is why he
mentioned "sprinkling" a few around.

But my real question is, aren't we interested in edge rates (edge
transition times), covered by the 0.1 uF decoupling caps, or do the slower
(assume periodic) frequencies of the RS-232 transmissions have an affect
which is different than and in addition to edge rate EMI and which needs
mid-range filtering such as the 6.8 uF caps (and I assume at those
frequencies the extra inductance is not a factor)?

I welcome more input from Steve and others on this issue of mid range
capacitors and how they are effective.

Thanks,
Shayle

At 05:42 PM 3/7/00 -0800, you wrote:
>If you are sadistic, they will drive your purchasing crazy, as tant's are
>hard to come by these days.
>
>The PLD edge rates need to be watched. You can optionally filter the
>outputs depending on the application, with anything from 10 ohm series
>resistors, to RC networks, should the edge rates pose a noise issue.
>
>The tantalum caps will filter only up to a couple of MHz. So the question
>that arises is, how much filtering do you need from at least a couple of
>KHz where your power-supply regulator impedance goes up to a couple of
>MHz? If you are really only running some RS-232 then the one 47 uF cap s/b
>plenty. The 100nF caps will go inductive around 8-15MHz ( mounted ). So,
>you will either need lots of them in parallel, or other smaller values to
>cover at least up to 80MHz. You can refer to recent postings on this
>thread for more details.
>
>Regards,
>
>
>Steve.
>At 06:20 PM 3/7/00 +0000, you wrote:
>>Hi,
>>
>>I have been asked to sprinkle 6.8 uF tantalum or electrolytic capacitors
>>evenly around the PC board of a new design.
>>
>>As usual, I have the 0.1 uF 805 ceramics at the power pins of chips and a
>>bulk storage cap. of 47 uF where power enters. The board is 3x4 inches, has
>>a few chips, slow edge rates. Mainly RS-232 (up to 1 mb/s) and the
>>internals of a PLD clocked at 16 MHz.
>>
>>Can anyone tell me what the benefit of these 6.8 tantalums sprinkled around
>>would be?
>>
>>Thanks,
>>Shayle
>>
>>
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