[SI-LIST] Re: Placement of decoupling capacitors
- From: Khalid Ansari <khalida@xxxxxxxxxxx>
- To: peter.flowerdew@xxxxxxxxxxxxxxx,"'mgreim@xxxxxxxxxxxx'" <mgreim@xxxxxxxxxxxx>,Larry Miller <lmiller@xxxxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Wed, 01 Aug 2001 10:00:52 -0700
Thank you all for your great inputs, they were very
helpful.
Khalid
At 01:50 PM 8/1/01 +0100, FLOWERDEW, Peter wrote:
>Hi Khalid,
>Your intuition is correct, but as other people have indicated, you have
>other options on your kind of pcb. However:
>
>I work on emc immunity of products, particularly against mobile phones- I
>frequently need something like 2n2 to cover below 80MHz and 15pF to cover
>around 1GHz - if you piggy-back the capacitors they work better than putting
>them side-by-side on the PCB. Two capacitors more than a decade apart in
>value will produce two notches in the attenuation curve. Syfer and Johansson
>offer this as a special packaged component, but it is not cheap. They also
>stack same-value components to produce high value low-inductance capacitors
>for compact switch-mode converters.
>
>You can also get what are called X2Y caps that are for decoupling a
>differential pair to a ground plane: C from line A to GND, C from line B to
>GND, and c/2 from line A to line B. Three capacitors in one package. The
>structure has currents flowing in opposite directions so the package is low
>inductance. Syfer and others have been selling these for motor suppression
>and I have been educating them to telecom applications. They are generally
>in 0805 packages, but I have some low values for hf in 0603.
>
>There is also a 'filter capacitor' which is a surface mount version of a
>feed through, but all it really is, is a low inductance capacitor. Low
>inductance is good - more farads at the same self-resonance or higher
>operating frequency for the same value.
>
>Just to show you the mileage in your idea: there is a special capacitor
>manufactured by AVX which has multiple resonances - The trick is that the
>resonances are at the frequencies used by mobile telephones so this one
>component acts as three tuned filters to remove the interference from
>phones. you can chose three of - 900MHz, 1.8GHz,1.9GHz and 2.45GHz.
>e.g. W1T15A248A - an 0603 component filtering 900MHz, 1.8GHz and 2.45GHz.
>
>Regards,
>
>Peter
>
>
>
>
>-----Original Message-----
>From: Greim, Michael [mailto:mgreim@xxxxxxxxxxxx]
>Sent: 01 August 2001 13:06
>To: 'khalida@xxxxxxxxxxx'; Larry Miller; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Placement of decoupling capacitors
>
>
>
>Hi Khalid,
>
>Well, I think that mfg would take great umbrage with
>piggy backing capacitors. For a prototype this might
>be possible but not for production. You might want to
>make use of plane capacitance to help at high frequencies.
>Think of what you are proposing, by piggy backing the
>caps, you are minimizing the foot print but not the loop
>inductance to the plane. Essentially you are just presenting
>a larger cap to the PDS. This will move your resonant
>frequency and give you perhaps unexpected behavior.
>
>You may get some additional decoupling through the use of
>smaller footprint caps like 0402 and via sharing with the
>bga if in fact there is no other way to get the decoupling
>in. I can't believe that the board is so packed, that
>there isn't a better alternative to piggy backing caps.
>If the board is that packed, routing will prove to be your
>next problem I suspect.
>
>Do you have a .brd file that I could take a look at and
>offer some suggestions on what options you might have.
>
>best regards,
>
>MG
>
>-----Original Message-----
>From: Khalid Ansari [mailto:khalida@xxxxxxxxxxx]
>Sent: Tuesday, July 31, 2001 5:11 PM
>To: Larry Miller; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Placement of decoupling capacitors
>
>
>
>Larry,
>
>
> >I think you have it right on placing the smallest caps (highest frequency)
> >closest to the pins, but these are very small (we use 0603's), so the next
>
>I just have enough space to place this one close to the pin, the rest have
>to go quite further away due to space constraint. Placing them too far
>away I have to worry about the loop inductance.
>
>Are there any issues with piggy backing capacitors?
>
> >bigger (lower frequency) ones are not that much further away. By the time
> >you get to the bulk caps, they only have to be in the general neighborhood.
> >
> >No tricks or piggybacks needed-- the lower the frequency range you need to
> >cover the less critical the placement. This is especially true where you
> >have power and ground planes that are essentially zero ohms at lower
> >frequencies.
> >
> >My opinion, anyway, and we haven't had any problems following this idea.
> >
> >Larry Miller
> >
> >-----Original Message-----
> >From: Khalid Ansari [mailto:khalida@xxxxxxxxxxx]
> >Sent: Monday, July 30, 2001 2:19 PM
> >To: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Placement of decoupling capacitors
> >
> >
> >
> >I looked through various emails from Larry Miller, Larry Smith and the rest
> >who had contributed to a discussion earlier on decoupling. Those
> >emails helped me select the values for decoupling caps but now the
> >challenge is, how do I place these? I don't have enough room on each of my
> >power supply pins to place these caps around. If I try to place the
> >cap that decouples the highest frequency components out of the selected
> >ones close to the power pin and the one that decouples the lowest frequency
> >components the farthest, the caps are being placed too far away from the
> >pin.
> >Can I piggy back these caps? What are some of the tricks to overcome this
> >problem?
> >
> >Thanks in advance,
> >
> >Khalid
> >
> >
> >
> >
> >
> >
> >
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>
>
>
>
>
>
>
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