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Why add bypass capacitors on a PCB like THIS?
Answering a Twitter question:
https://twitter.com/furan/status/1586889993234038786
Are bypass capacitors really needed? https://www.youtube.com/watch?v=P8MpZGjwgR0
Bypass capacitors visualized: https://www.youtube.com/watch?v=1xicZF9glH0
Bypass capacitor tutorial: https://www.youtube.com/watch?v=BcJ6UdDx1vg
https://www.vgamuseum.info/index.php/cpu/item/993-s3-supersavage-ixc-sdr
https://docs.xilinx.com/v/u/en-US/wp411_Sim_Power_Integrity
Forum: https://www.eevblog.com/forum/blog/eevblog-1512-why-bypass-your-pcb-like-this/
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#ElectronicsCreators #Bypassing #pcb

By YTB

25 thoughts on “Eevblog 1512 – why bypass your pcb like this?”
  1. Avataaar/Circle Created with python_avatars SystemX1983 says:

    Looks like a cemetery for electrons 😯😂

  2. Avataaar/Circle Created with python_avatars Nita Vesa says:

    The tablet setup is working peachy, but the instructor is a little wonky! 😉

  3. Avataaar/Circle Created with python_avatars Steve Robbins says:

    This was a designer who didn't know how to utilize bypass caps effectively, so he just sprayed the board with them. Sad.

  4. Avataaar/Circle Created with python_avatars Haskellerz says:

    RTX 4090 has more caps than this

  5. Avataaar/Circle Created with python_avatars Aiken Drum says:

    carousel loading

  6. Avataaar/Circle Created with python_avatars tim un says:

    putting your caps on the bottom will not always give you better performance, if you have a tightly coupled plane pair close to the populated side of the board having your caps somewhat far away on the same side can be better than having long inductive vias, this is not what is happening with this board but i wanted to mention it anyway

  7. Avataaar/Circle Created with python_avatars LqqkOut says:

    Thanks for the board tour! The pen/tablet seemed to work well!

  8. Avataaar/Circle Created with python_avatars Randall jones says:

    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.

  9. Avataaar/Circle Created with python_avatars TheCod3r says:

    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

  10. Avataaar/Circle Created with python_avatars ThatGuy Makes Things says:

    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.

  11. Avataaar/Circle Created with python_avatars Gordon Welcher says:

    A thing of beauty and a joy forever.

  12. Avataaar/Circle Created with python_avatars IcemanFPV says:

    Looks like it was designed by a female with autism.

  13. Avataaar/Circle Created with python_avatars Bobby says:

    Thanks Dave,
    it would be good to see those bypass capacitor computer model programs in action.

  14. Avataaar/Circle Created with python_avatars Kyan K says:

    Its look that way as it was made in gods image, god being the a.i. That govens this sim we run in.

  15. Avataaar/Circle Created with python_avatars Jammit Timmaj says:

    I make everything in a grid except for one part that's laid out, intentionally, slightly skewed. I'm silly.

  16. Avataaar/Circle Created with python_avatars Dan Bowkley says:

    I'm starting to wonder if Bruce Dickinson moonlights as a PCB designer

  17. Avataaar/Circle Created with python_avatars ptstv says:

    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…

  18. Avataaar/Circle Created with python_avatars Soniclab says:

    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”

  19. Avataaar/Circle Created with python_avatars Lindsay Wilson says:

    What software are you using to let you do the live drawing/annotation? Need something like that!

  20. Avataaar/Circle Created with python_avatars Vincent Himpe says:

    most likely that super savage has multiple power domains and they may be staggered. so they will be alternating 5v and 3v3 decoupling caps. A lot of the other capacitors have resistor chains nect to them. look at the 25 pin connector, you see traces running from one resistor to the next. those are most likely impedance matching / low pass filters for tha analog i/o (video signal). the same goes aroudn the focus chip and between the focus and savage chip. those are most lilely matching networks / lowpass for video

  21. Avataaar/Circle Created with python_avatars Hola! ^Gecko^ says:

    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.

  22. Avataaar/Circle Created with python_avatars Andrew Young says:

    Would the part placement not have been automatic for something like this? (I.e. not just the routing)

  23. Avataaar/Circle Created with python_avatars Hola! ^Gecko^ says:

    wtf, my comment disappeared?

  24. Avataaar/Circle Created with python_avatars Tiago Ferreira says:

    Maybe many capacitors are being used for signal conditioning (coupling/decoupling/filtering…) since VGA is analog. Just a hunch.

  25. Avataaar/Circle Created with python_avatars Kyle LeClair says:

    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.

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