Decoupling or Bypass Capacitors, Why?

Discussion in 'General Electronics Chat' started by thatoneguy, Nov 14, 2010.

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  1. thatoneguy

    Thread Starter AAC Fanatic!

    Feb 19, 2009
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    I posted something like this a while back, but I've noticed a good number of newer members forgetting the bypass/decoupling caps in designs, or asking about their purpose in life (the caps, not the members).

    The reason for them is "stiffening" of the power supply. The wires on a breadboard, extraneous bus lines on a solderless breadboard, etc all create parasitic resistance and inductance. This resistance and inductance (impedance) of the board resists current flowing from the power supply to the microcontroller or logic IC.

    Here is where the bypass capacitor acts like a mini power supply to "hold up" the voltage until the current overcomes the impedance of the power supply lines to feed the IC. I put 3 0.1uF down each edge of the solderless breadboard, spaced evenly, 1 at each edge and one in the middle, for a total of 6. These are a permanent fixture on my solderless breadboards.

    As for "Why electrolytic and ceramic in parallel?": Electrolytics have high storage capacity for their size, but this comes at the cost of higher Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). These are the same parasitic elements of the circuit board we are adding the capacitor to correct! Hence, the mica or <insert dielectric here> cap that has nearly zero ESL and extremely low ESR. The smaller cap is able to supply the needed current instantly to "cover the gap" until the Electrolytic can "fill the gap" until the power supply finally "catches up" after a switching operation in a large logic IC, which uses a lot of current, relatively.

    The other way of looking at these capacitors is as mathematical models, essentially broadband filters to "bypass" any ripple directly to ground.

    I've explained this to a good number of people, and the one I expounded on most (mini-battery concept) is the one people tended to understand where I witnessed new understanding and enlightenment. This is mostly why I didn't cover the filter concept. The other reason is it gets ugly. Most here know capacitors pass AC, but that isn't good enough. In order to fully explain the principles, example values would need to be set up showing the PCB as a transmission line, and it would end up on a Smith Chart before the arguments were started.

    I found this website below to be an excellent source of info on why all designs that use pulses (logic, 555 timers, etc) need a bypass cap across the power pins.

    Capacitor Design Tips for PCB

    It also has information on which type of cap to use and why. It also explains the reasoning behind the common usage of a 22uF Electrolytic in parallel with a 0.1uF Ceramic.

    When working on circuits that have external signals in the Khz range, there really isn't much else to add as far as circuit design. When the clocks get higher though, there are entire seminars and rituals on moonless nights for PCB Design, divining layers, what to put on which, where vias can be, where to never put them, etc. So I'm just sticking with the caps on this post, as they solve a lot of problems, and not many people are making more than dual layer boards as a hobby anyway. :D

    Finally, when it comes to any circuit, analog or digital, always remember that performance will ONLY be as good as the Power Supply.
     
    Last edited by a moderator: Nov 26, 2010
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Good idea and useful, but unfortunately, it will soon be buried in other chat. A sticky? An addition to the e-book? An edited version of FAQ's or "Electronic Tips and Tricks?"

    John
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    We really do need a "sticky" or E-book entry on this very subject, as it applies to virtually any IC or circuit that one could think of. The more experienced folks know that every IC needs at least one 0.1uF/100nF "supply bypass" cap, and many datasheets recommend more than one, such as the 555 timer; it needs at least one 0.1uF poly metal or ceramic and one 1uF aluminum electrolytic or larger in parallel.

    Many "n00bs" will put together a circuit exactly as shown in a schematic, and it either doesn't work, or works very poorly! Frequently, bypass caps just are not shown at all on a schematic, or they are shown down a bottom corner all connected in parallel to help eliminate "clutter". After all, everyone knows that bypass caps are needed on EVERY IC, right?

    Not if you're new, and don't know that you need to read the datasheet(s) for the IC(s) being used. New Folks, these power supply bypass caps are NOT optional. If you do not include them, you will have odd problems, if the circuit works at all.

    If the IC in question is a single-rail supply (eg: ground and Vcc or Vdd) then you need at least one 0.1uF/100nF metal poly film or ceramic cap.

    If the IC has a dual-rail supply (eg: Vcc/Vdd, GND, and Vee/Vss) then you need at least two caps; one from GND to Vcc/Vdd and one from GND to Vee/Vss.

    All logic IC's (74xx, 4000 series) require one 0.1uF/100nF cap across the Vcc/Vdd/GND supply right at the IC itself. MCU's/uC's also require at least one bypass cap. If you have heavily loaded outputs, you may need more than one cap.

    When you are "breadboarding" a circuit, try to use the shortest jumper wires possible to connect your components, as long, loopy wiring will have a LOT of inductance, with plenty of potential for perplexing problems, particularly with digital circuits.
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I'm stickyin' it right now into notepad and saving it. Of course, this too will end up burried in some folder on some hard drive on one of many computers. Maybe a refrigerator magnet!!


    iONic
     
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  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    That's a Great idea. Let's start a fund for contributions to support it. Every new member will get a refrigerator magnet with rules for decoupling capacitors. It could begin, "We hold these truths to be self evident."

    John
     
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  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Okay, This will get stuck and closed so it doesn't founder under the zillions of comments.
     
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  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Decoupling is always a basic need.
    The attached PDF will also give good information for RF prototyping.

    Bertus
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here I found an other PDF on decoupling capacitors.
    The PDF explains that capacitors are not ideal and what the decoupling response might be.
    It also explains why you will see two capacitors in parallel in the circuit.

    Bertus
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here I found an application note with the title:
    An IC Amplifier User’s Guide to Decoupling, Grounding,
    and Making Things Go Right for a Change

    Bertus
     
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  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here I got two more PDF's on the effect of non ideal capacitors and their position on the PCB.

    Bertus
     
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