Re: [SI-LIST] : Medium range capacitors

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From: sweir (weirsp@a.crl.com)
Date: Tue Mar 07 2000 - 19:21:50 PST


Shayle,

Sure, if you use enough of them, 100nF caps work ok to high freq, but they
are acting as parallel inductors at high freq. The papers delivered by
Larry Miller and co. of SUN show that for demanding applications, careful
choice of a wide range of capacitor values provide a lower impedance at a
lower component count than is possible either with many 100nF or similar,
or capacitors on decade or century boundaries.

The mid frequency performance is just a matter of impedance
calculation. From 1KHz up, the 47uF capacitor starts life at approximately
3.5 ohms, and will bottom out at the combined ESR, ESL, which depending on
the part will vary from the 10's to 100's of milliohms. RS-232 currents
depend on the voltage swing. The typical today is +/-5V or less. That
will yield about 100mA max on a mark-space excursion. If your noise budget
is say 100mV, then one line requires a bus impedance of an ohm or
less. How far down the spectra extends depends to some extent on how long
the RS-232 lines are. Even if you have 100' of cable, the current will
drop-off within one round trip at about 500nS, so even the low frequency
stuff is still in the hundreds of KHz. After that, it is literally the
difference in DC load, and RS-232 lightly loads, so this is very small.

Over the space of a 3 inch board a wave will propagate across the whole
board in less than 1 nS. Bulk capacitors respond on the order of 100's of
nS, so distribution doesn't buy anything. The ceramic capacitors and the
planes are doing all the work. If I were your customer, I would pay a lot
more attention to the PLD than the 232 drivers.

Regards,

Steve.

At 08:44 PM 3/7/00 +0000, you wrote:
>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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