The Tee connection would add series inductance, in addition to that of the =
cap itself, lowering the self resonant frequency.  The better the cap and g=
round system, the more difference the routing makes.  Johanson X2Y caps are=
 pretty good caps.

I did an SMPS design years ago where I saw a 2dB reduction in EMI by routin=
g the track from the output diode to the capacitor lead (thruhole cap) to t=
he output rather than routing those three points as a small flood, which al=
lowed current to take either path.
I usually do very well on radiated emissions, in the "are you sure it's on"=
 domain rather than "well, it's probably going to pass", by using the track=
s I must have to work for me rather than against me.


-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 Ryan O'Connor
Sent: Monday, December 4, 2017 1:48 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Bypass capacitor really required?

I have a question about this... is there really a difference between "route=
 so that power goes THROUGH the capacitor pad on its way to the chip" and n=
ot? Does anyone have empirical evidence of this working vs not? Or is it ju=
st something people have imagined?

Just curious as have not seen any tests around which prove the need for it.

Ryan




On 5 December 2017 at 07:16, Van Horn, David < david.vanhorn@backcountryacc=
ess.com> wrote:

> Plenty of good answers, and I'll throw in a "HELL YES" as well.
>
> I saw one instance of a product in production using an Atmel AVR,=20
> where there is a single pin which is an ADC AREF input only rather=20
> than a fully implemented I/O pin.
> The application didn't use the ADC at all, and the designer thought he=20
> didn't need that bypass cap.
> There was a box of boards which had failed production test, which had=20
> resisted all attempts to repair.
> I added the specified bypass cap and recovered 100% of those boards.
>
> If the data sheet specifies bypass caps, design them in.  If it=20
> doesn't, design them in anyway, you can always DNP (do not populate) in p=
roduction.
>
> Bypasses are your friend.  Route them well, and don't skimp.   I use X2Y
> caps in critical applications. With any type of bypass cap I route so=20
> that power goes THROUGH the capacitor pad on its way to the chip, and=20
> the ground side of the cap returns directly to the nearest ground pin on =
the chip.
> Never a "tee" where the current has the option to go past the cap.
>
> Similarly with crystal loading capacitors, where I route them directly=20
> to the nearest ground pin on the chip and nothing else touches that=20
> trace until it joins all the ground pours at the chip ground pin.
> I have seen boards fail FCC testing hard because crystal caps were=20
> "grounded" into a 100 mil ground track that was about a quarter=20
> wavelength long at 400+ MHz.  The "ground" actually worked more like a=20
> shunt fed antenna. :-P
>
>
>
>
>
> --
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> View/change your membership options at=20
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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