Agreed, but they aren't actually separate items. More of a case of degree.= =20 When you have to pass RF susceptibility testing and ESD testing and fast tr= ansient testing as well as emissions very good bypassing is probably the le= ast expensive way to get there. I've had bare two layer circuit boards pass susceptibility testing at 192V/= m (the measuring equipment couldn't stand more, my board was fine) I've not had issues with ESD or fast transient testing. I'm an absolute sadist with testing, it has served me well for a long time. ESD testing can be done with the Schaffner gun, but this is a lot more fun:= http://sciedwiki.pppl.wikispaces.net/ETP+High+Frequency+Generator 50kV at 3-4A, risetime about 30nS. -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of= Sean Breheny Sent: Monday, December 4, 2017 2:30 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] Bypass capacitor really required? I do use this technique but I have not done actual testing to determine how= much difference it makes. However, on paper it looks beneficial. Bear in mind that we are really talking about two different reasons for byp= assing here: circuit reliability and EMC (electromagnetic compliance). David's technique is really mostly about the EMC part. Depending on the exa= ct circumstances his technique may help or slightly hurt the circuit reliab= ility part. It can almost always be done in a way which doesn't negatively = impact circuit performance. David's technique is about minimizing the amount of high frequency current = flowing in paths which are long enough to have significant radiation resist= ance (i.e. "make good antennas"). Such a path need not be in a trace - it could even be in a plane. The circuit performance aspect focusses on minimizing the impedance (and th= e Q factor) of the power and GND nodes as seen by the chip, so that supply = current pulses do not cause droop or "ground bounce". Sean On Mon, Dec 4, 2017 at 3:48 PM, Ryan O'Connor wrote: > I have a question about this... is there really a difference between=20 > "route so that power goes THROUGH the capacitor pad on its way to the=20 > chip" and not? Does anyone have empirical evidence of this working vs=20 > not? Or is it just something people have imagined? > > Just curious as have not seen any tests around which prove the need for i= t. > > Ryan > > > > > On 5 December 2017 at 07:16, Van Horn, David <=20 > david.vanhorn@backcountryaccess.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=20 > > he didn't need that bypass cap. > > There was a box of boards which had failed production test, which=20 > > had 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)=20 > > in > production. > > > > Bypasses are your friend. Route them well, and don't skimp. I use X2= Y > > caps in critical applications. With any type of bypass cap I route=20 > > so > that > > power goes THROUGH the capacitor pad on its way to the chip, and 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=20 > > directly to the nearest ground pin on the chip and nothing else=20 > > touches that trace until it joins all the ground pours at the chip grou= nd 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 > wavelength > > long at 400+ MHz. The "ground" actually worked more like a shunt=20 > > fed antenna. :-P > > > > > > > > > > > > -- > > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive=20 > > View/change your membership options at=20 > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive=20 > View/change your membership options at=20 > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/chang= e your membership options at http://mailman.mit.edu/mailman/listinfo/piclis= t --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .