EEVblog 1512 - Why Bypass Your PCB Like THIS?

Why add bypass capacitors on a PCB like THIS?
Answering a Twitter question:

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#ElectronicsCreators #Bypassing #pcb

[kyleleclair]

I’ve been watching your videos Dave for 8 years. Ever since I was a young Marine working as an aviation electrician. Through the knowledge you taught me and applying that knowledge in the field I became Marine of the Year and I was able to pass the screening for a EE R&D job while grossing 6 figures without any college education. So I’m working and studying biophysics living the dream in my new home. Thank you Dave for being a crucial pillar in my professional development.

[henrychan]

"2004 is almost 20 years ago"
Shit sure didn't feel that way

[starlite]

This card was made by Integral Technologies as a video capture card for 16 or 32 channels of analog CCTV feeds. It would have connected to an external input box via the large connector on the card. A lot of the people that worked for them now work at Exacqvision.

[UpLateGeek]

Not sure how expensive these boards would've been back in the day, but if they were pricey enough, they may have wanted to leave clearance around the BGA chip for manual rework. Like if you've got $100 worth of chips on the board and the only thing stopping it from working was a dodgy BGA joint, it could be worth a manual reflow, or possibly even removing the chip reballing and resoldering. That would save you from eating a $100 loss (plus the margin of that board).

[cavemaneca]

As someone designing a product with a high end modern processor, I WISH the manufacturer had supplied us with a 50 page document on power timing. We're lucky to have a couple pages with 2-3 diagrams.

[HerrSMINI]

Lying in bed 2am in the morning in Germany. Can’t sleep. Enjoying your videos.

[Novous]

"I thought it was a card from the 80's"
> shows card with PCI bus

ಠ_ಠ

Have a great day, love the vids.

[TheCodr]

Great explanation Dave! As a repair technician these types of videos do massively help to understand why things work the way they do. Keep it up bud

[MrJohnBos]

I love the concept of using a drawing pad overlay on the circuit board. Once you get it all figured out, it will be a really great tool.

[JamesHalfHorse]

I would assume from all the I/O it's probably a broadcast grade card and if it's like my broadcast audio cards it has a lot of extra shielding and noise reduction. Working in studios that share close proximity with the transmitters noise and interference issues a constant. I haven't quite seen it quite like that but I wouldn't be surprised if I did.

[robroysyd]

Perhaps all those caps were put there to reduce RFI into the analog video inputs keeping in mind that not everyone correctly terminated their video feeds. The other thing is those caps would have been pretty cheap and it does make the board look cool. Given the choice potential customers are likely to go for the board that looks full.

[shaun]

Kudos to whomever for squishing all those caps on one side though.
Must have been satisfying when the last track snapped in place and they zoomed out to admire the rows and columns.

[demnk]

It's a pretty full layout. So well done to the designer for getting it all to fit top side. The crystal routing is a bit 'how ya doin?' though :D New setup looks great Dave.

[Tomato]

PCI cards that have two notches in the connector can accept both 3.3 and 5V, though most most desktop mainboards have 5V slots (indicated by the notch away from the bracket)

[RS-lsmm]

Probably didn't read all of the manufacturers decoupling guidelines. Maybe read the required number/type of caps but not how to arrange them. Happens a lot when the circuit designer and the board layout designer don't talk to each other.

[teardowndan]

It is an analog input board. Having a row of bypass caps surround digital components could be an attempt at "fencing in" digital noise by providing a very low impedance path between ground and power planes around and between the chips.

[ptstv]

Hard to say without looking at the schematic, but if there are a lot of channels of video with parallel busses, depending on the format and bit depth, you could have a lot of simultaneous switching currents. Especially if the channels are genlocked. With a mixed signal board like this, you'd want to keep things like ground bounce to a minimum. I could see wanting a lot of caps on board, although not sure that many...

[randalljones]

A couple thoughts on this :
a) The system designer started out the schematic capture phase by dedicating one page to caps (decoupling and bulk) and just cut/paste a metric tonne of decoupling caps on it... maybe a post-pub edit session?
Since all the challenging stuff happens on the OTHER schematic pages (and who brings up the cap page???) the over-abundance slipped through.

b) They used some rather new-for-the-time chip and were unsure how it would behave, so they over-populated the board with decoupling caps.... since you CAN leave some off and suffer little penalty (at those frequencies and transition currents) they were going to test how much difference they'd have between the over-populated board vs one with 3-4 caps per BGA.

c) Are all the small yellow-bodes the caps, and the grey-bodies resistors? If so, much of what we are looking at may belong to some series termination scheme. I'm not a video designer, but it is (or was) a fairly common thing to do, and if they have a bit of an odd stackup for trace impedance (e.g. their systems engineer is a repurposed software guy who isn't the most pcb-cognizant of people) they might have mad to do a little catchup impedance matching.

d) This could be an example of off-site/off-country PCB design where the wording of the pcb spec was a little ambiguous, and the pcb-house wasn't too keen on asking questions... they just follow a spec.

e) some smart-child wanted to see what happens to a nice quite set of power planes edge-to-edge inductance if you blow it full of holes.

f) Is this driving a standard display family? This isn't for some old high-voltage plasma display, is it?

g) In review, I'm probably more in the camp with the folks who say this boards got multiple power rails because of a mixed-signal function. And they want to decouple away power-ground spikes caused by some noisy WIDE logic bank (> 64 simulanteous bit transitions) and a higher-impedance analog side (subject to pickup spikes). .. and an overzealous noise-budget engineer.

[johnwest]

I could tell a lot more by looking closely at the board and component selection, but I'll guess it was a brand-new start-up that hired a young design engineer right out of school or with limited experience and put them in charge of the board design without an experienced supervisor. It clearly has a bunch of I/O, so in order to cover their butt the engineer told the layout person to decouple absolutely everything. With the quantity buying power they got from all of the caps on this board they may not have even lost much money on the deal. There are often back-stories on board designs like this one. Also, it looks like the company may be a home-monitor and control company, and I've seen some boards from such companies that were definitely belt and suspender designs, practically aerospace grade, and priced accordingly.

[soniclab-cnc]

Likely they took all the effort to keep capture quality clean. Any ripple in the video signal encoding. I’ve seen less decoupling in high end video equipment lol. They just went all in for this one. “If there is any interference it’s not my fault”