Decoupling caps again

Discussion in 'General Electronics Chat' started by spinnaker, Dec 12, 2010.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I dutifully read this thread regarding decoupling caps . In the thread Sarge says "require one 0.1uF/100nF cap across the Vcc/Vdd/GND supply right at the IC itself".


    I know the cap should be as close to the VDD as possible, But does the above mean i needs to go to the ground of the chip?

    For example, on my PIC VDD is pin 1 and VSS is pin 20. Should the cap really go from pin 1 to pin 20 or is going to ground from pin 1 ok?
     
  2. SgtWookie

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    If you have a ground plane on your board, you can connect the cap to pin 1 and the ground plane.

    If you do not have a ground plane, you will need to connect one end of the cap to pin 1, and the other end to pin 20. The wire leads should be as short as possible.
     
  3. retched

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    I often use the bottom of the PCB for my decoupling caps.

    That way I dont need to "straddle" the IC to get to the GND pin.

    Tack them to the vcc and GND pins that protrude on the back.

    When you get to be as good as me ;) (hahaha), you can slide the pins in the same hole and solder at once.

    For devices that have the GND and VCC pin next to each other, or when you have a ground plane on another layer, I use SMD caps for decoupling and layout from VCC to a GND-plane VIA.

    This makes for VERY short leads on the cap (obviously) which means less inductance and it gets you used to SMD soldering.

    Get a bag of 100 .1uF through-hole, and a bag of 100 SMD 0805 .1uF caps.

    The 0805 size SMD caps are sized to fit between 2 through hole pads.

    For under $10 including shipping, you can have 200 decoupling caps that will cover most everthing you come across.

    One important thing to keep in mind.. DFM. Design for Manufacturability.

    Be sure if this is going to be built by a Pick-and-Place machine, you will want to place pads for the SMD caps and have a small trace connect to VCC and GND.

    When prototyping or making one-offs, soldering the 0805 directly to the pins is nice and easy.

    I use tweezers, or a chop stick with some adhesive junk applied to the end. It is 3m tack goo. I no longer have the packaging insert.

    It comes in white and blue and it releases the chip well. I use it for holding headers in place too now, thanks to advice on this forum.

    A drop of super glue between the chopstick and the adhesive paste stuff, keeps the adhesive from pulling off of the chopstick when you are done.

    Mouser:
    0805 .1uf caps. $2 per 100.
    http://www.mouser.com/Search/ProductDetail.aspx?qs=VOOUd%2bza08qHu13WgNByHQ==

    Through-hole .1uf caps $4 per 100
    http://www.mouser.com/ProductDetail...EpiMZZMuAYrNc52CMZOqBXleu%2bdrVRNc1e%2baSdJw=

    You should be able to get shipping around $4 or so.
     
  4. spinnaker

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    Thank you both,

    Great idea on tacking the caps on the back retched!

    How would you handle a 16 pin chip where vcc was pin 16 and gnd was pin 8? They are across from one another. :) Just use some heat shrink?
     
  5. retched

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    Heat shrink? I am having trouble picturing this...


    However, if the pins are far apart, you do not HAVE to go to the ICs GND pin. Just a GND.

    This is why ground planes are so convenient.

    The nice long leads on the through-hole caps can reach . go under the iC on the bottom of the board.

    The green in on the SOLDER side. Black is COMPONENT side.
    For diagonal:
    [​IMG]
    And across:
    [​IMG]
     
    Last edited: Dec 12, 2010
  6. spinnaker

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    Pretty much as in this pic except heat shrink on the leads. I will have runs going under the chip so a cap stretched across like this will short.

    Here is where I have my issue. On U2. C4 is my decoupling cap.
     
  7. retched

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    Make the leads as tight as possible.. Like nice wires on a breadboard.

    [​IMG]


    If you are running any traces under the IC on the bottom, then the shrink wrap would be a good idea.

    A few swipes of solder mask (fingernail polish) will also work.

    The cheap fingernail polish is not lacquer.. It wont flake off. It is like latex paint. I get my fingernail polish at the $1 store. (As well as many heat sinks, and interesting project cases) (And I look FABULOUS in it ;) )
     
  8. bertus

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    PackratKing likes this.
  9. mtripoli

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    Feb 9, 2010
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    One thing I'd like to point out on the subject of decoupling caps and PCB layout. It's a common "mistake" and often overlooked. The power pin (Vcc, Vdd, etc.) should only be connected to the cap, not the "power" rail. The difference is subtle but can make a huge difference in the performance of a circuit (I know it's not common for hobbyist and even some pro's to do low noise high gain amp designs) but if you are aware of it it is a simple thing to do and your circuit will be the better for it. Looking at the attached PCB example you'll see the power rail trace (highlighted with a white line around it). The power rail goes to the cap and then a very short trace goes from the cap to the supply pin. No other "pin" is attached to the this trace.

    Mike T.

    EDIT: The layouts shown are for example. In the first one the design constraint was that all components had to be on the top side of the PCB. If this was a "critical" design the caps would be on the bottom right next to the supply pins. The middle picture shows a correct layout, the third a not so great layout.
     
    Last edited: Dec 13, 2010
  10. mtripoli

    New Member

    Feb 9, 2010
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    Neat link, and they got it *almost* right :)

    In the animation they show the power supply rail going to the power pin with the cap "hung" off the pin. The lead should go to the cap with a short trace (in practice the cap goes so close to the lead there esentially "is no" trace).

    Again, it's subtle, but in some circuits where PSRR and CMRR are important (I've designed amplifiers where I had to pull single "electron events" out of a vacuum and every little bit counts). If you get in the habit of doing it it's not a big deal.

    Mike T.
     
  11. thatoneguy

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    Feb 19, 2009
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    Preferred way would be a trace/flat wire between pwr and ground leaving a small gap. A flat trace has less inductance and EMI susceptibility than a round wire. Move the traces together so a SMD cap, or a standard cap can be used with very short leads.

    Doing this also obliviates the need for insulating the bare wire. In some designs, it is unavoidable as too many other signals are routed under the IC in question, but if you have the room, try to have the traces make most of the length, rather than using the very end of the long leads on a small cap.
     
  12. SgtWookie

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    This is where multilayer boards come in handy.

    If you have one layer for a ground plane, and one for Vcc/Vdd, you can sprinkle decoupling caps around and wind up with a very quiet board.

    Even a straight piece of wire has inductance. A ~10mm long piece of paper clip has ~15nH inductance. I discovered this a number of years back when an engineer was using a paper clip to calibrate a $20,000 material analyzer; and it was causing a ~15nH offset in all of my readings.

    The wider a trace is, the less inductance it has. An infinite plane has virtually zero inductance, which is why they use power and ground planes on circuit boards.
     
  13. spinnaker

    Thread Starter AAC Fanatic!

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    Thanks for the tip on nail polish. What is your favorite color? Red maybe? Or does that make you look too trashy? :)
     
  14. retched

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    Haven't you seen my photo? It is IMPOSSIBLE for something like fingernail polish color to affect that kind of gorgeous! ;)

    I use HOT PINK.

    The best I have found is a type for kids that is edible.. or at least non-toxic.

    If kids bite their nails, this polish is "ok" to ingest.

    You can PEEL it off of your fingernail, it doesnt "flake" off, and adheres well to fr4 and the traces. I have done a bunch of tests for breakdown and conductivity, and I have had no problems. In 12v 350mA traces, I had no problems what-so ever. The coating went from bare-tinned traces, and covered hot and GND and had no continuity AFTER DRY.


    Every new bottle you get, draw a line a few coats thick, let dry for 30 min or so, then test for continuity & resistance. If it is conductive, then you can use it for your next RAVE party costume. ;)
     
  15. thatoneguy

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    Cool tip, Retched! What brand, specifically, is at the $1 store?

    Does it come in a green close to standard solder mask green? I've used Testors 1601 Transparent green spray can to make boards look better after putting in a new trace due to trace becoming a fuse.

    Second choice (when making a new DIY board) is vitrea 160 Tea Green (#15) from pebeo.com (needs brush or airbrush).

    Both of those don't conduct (wet or dry) and make a half decent solder mask, but cost more than $1, so I am interested.

    I don't go to many Raves, so I'd like to be forced to test as few as possible. ;)
     
  16. retched

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    I dont know.

    The polish was not labled. It had the color label on a small sticker on the bottom. It was in a cardboard display case that had 30 holes in it to fit 30 bottles.

    It said "eatable nail laquer" chinglish

    Yes, they have many many colors. The forest green, lime green, and green green are all pretty close to board colors.

    Warning: "eatable chinese nail laquer likely contains high levels of lead"

    ;)
     
  17. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Recently reading the posts about placement of decoupling capacitors as well as looking at other poster's PCB designs, I was hoping someone would comment on my PCB board. It has been manufactured and is working for me, but I realize that it could be improved further. It is for a hobby application.

    For example, I've learned that in the future, I should also include decoupling capacitors at my power input, as well as at each IC. I can also see that I could have placed the capacitors C1 and C2 closer to the power connection of the LM386 and with some shuffling of C4, also closer to the LT1013.

    Second, I placed circuit traces on the component side wherever possible, soldered on the reverse and the remainder of the solder side was a ground plane. Some of PCB designs I've seen here have this reversed. Is there a preferred side is for traces versus the ground plane?

    [​IMG]
     
  18. count_volta

    Active Member

    Feb 4, 2009
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    Here is the board I'm using for my project. By the way I highly recommend it. Soldering on it is so easy!!! You touch a point, and done. Touch and done. Try this with Radio shack boards. Ewww. I got so angry at the RS board, I threw it across the room and screamed. Australia is leading the way in protoboard design as far as I can tell.
    Buy it here. http://www.protostack.com/index.php?main_page=product_info&cPath=1_2&products_id=1

    [​IMG]

    So the ground plane would be the white strip or bus? I am using the red strip for power. White strip for ground. My pin 14/16 is very close to the white strip. I can just connect a capacitor from 14 to the white strip.
     
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