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Choosing & Using Bypass Capacitors

Even though we publish new blogs daily from your intrepid editor and from a number of well known writers, sometimes it's good to have a look back at some of our well received articles from the recent past. Here is one that discusses bypass capacitors: Why you need them; what happens with your circuit if you don't use them properly; how the capacitors are constructed; and the characteristics of different types of capacitors.

The article was written by Mike Wong and Tamara Schmitz, two people with whom I worked at Intersil. They are both very smart, very knowledgeable folks who know about these topics. Since writing this article, Mike has moved on to Semtech. Tamara is still at Intersil and has an article over on EDN called An engineer on safari – what African animals teach about problem solving.

15 comments on “Choosing & Using Bypass Capacitors

  1. antedeluvian2
    February 27, 2013

    Brad

    Clicking on the link to the original article works fine, but clicking on the link to part 2 within the original article brings me back to this blog.

    Same is true for the link to transmimpedance op-amps.

  2. Brad Albing
    February 27, 2013

    D'Oh! Let me go fix those busted links right now. Thanks for spotting that.

  3. patrick_m
    February 27, 2013

    Thanks for flagging that, antedeluvian! It'll be fixed pretty quickly, but in the meantime, the search box is actually pretty good. A quick search of the headline of part 1 gives you all you need, and more, including the best articles of 2009: Enjoy!

  4. Bill_Jaffa
    February 27, 2013

    I worked with Tamara and Mike on getting this article posted and published. We were concerned that devoting so much length to such a mundane topic would be a losing proposition. Fortunately, pages on the web are not limited, so we said “why not?” and was posted despite the length

    Turns out this was the most popular article by far (in page views) for several years on the site (all parts of it!). Lesson to remember: so-called “dull” topics are actually very important to a successful design and product release, despite what the processor/software people would like you to believe! And since the article was not written by a capacitor vendor, the audience felt it had a non-biased perspective.

  5. eafpres
    February 27, 2013

    This material is very useful, and reminded me of another application of capacitors–using a blocking cap in-between an antenna and the radio front end.  Sometimes it can be part of a small matching network if the antenna isn't inherently matched to the feed.  This approach is very useful to block some ESD from entering the electronics through the antenna.

  6. amrutah
    February 27, 2013

    ESR will cause a voltage drop across it, at high currents, and the effective voltage across the capacitor will be less. It adds a zero at some frequency. What is the effect of ESL?  For onchip bypass capacitors do we have to consider ESL??

  7. amrutah
    February 28, 2013

    Bypass Capacitors are useful for filtering the noise and help improve the PSRR and the transient needs of the circuit.  Sometimes this is referred to as decoupling capacitors.  The decoupling capacitor works as the device's local energy storage for high transient currents. The capacitor cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current demands.

      Given the advantages, the derating of the capacitors can be issue and the reliablity of the product, so choosing good capacitor is a big thing…

  8. Brad Albing
    February 28, 2013

    Exactly right regarding local energy source. The cap need only supply short term narrow spikes of current. As long as the ESR and ESL are low, it works as needed. At the higher frequencies (frequency domain) – or higher di/dt (time domain), that pesky ESL starts to dominate.

  9. Brad Albing
    February 28, 2013

    Oh – just a little more on that topic: Since the inductance is on the same imaginary axis as the capacitance (pesky complex terms!), but opposite sign, the inductance starts subtracting away the capacitance. At some point, the capacitance is reduced to zero and then starts to lok inductive with higher and higher frequencies. That's waht is shown in Figures 8 – 11 in the original article.

  10. SunitaT
    February 28, 2013

    Bypass Capacitors are useful for filtering the noise and help improve the PSRR and the transient needs of the circuit.

    @amrutah, true. While capacitors are able to resolve noise issues, they can be vulnerable to parasitic resistance and inductance, with performance affected by temperature, voltage, mechanical effects, and other factors. The wrong choice or positioning may actually create added noise, power losses, or unstable circuit behavior.

  11. antedeluvian2
    March 1, 2013

    Although only slightly off topic, I thought I should mention a great article on ceramic capacitors (http://www.edn.com/design/analog/4402049/Temperature-and-voltage-variation-of-ceramic-capacitors–or-why-your-4-7–F-capacitor-becomes-a-0-33–F-capacitor)  from Planet Analog's (and Maxim's) Mark Fortunato

  12. Brad Albing
    March 4, 2013

    Yep – Mark's article is a good one for reference. Thanks for pointing that one out.

  13. TamaraISchmitz
    March 14, 2013

    Depends on the frequencies present in and around your system.

    I only talked about ESR in the article because it was easier to relate to.  ESL is just as important when high frequency systems are involved. 

    You can choose caps with low ESL (look at datasheets) just as you would look for ones with low ESR.  These will work in ALL situations.  However, as you might suspect, they will be more expensive as well.

    Another “trick” is to put multiple copies of capacitors in parallel.  This will put their ESL values in parallel as well–reducing the overall effect.

  14. amrutah
    March 15, 2013

    @Tamaral:  Using parallel capacitors will help reduce the parasitic resistance and inductance, but will impact the area on board… and also since the no. of components increase will directly result in increase of cost…

    Please correct me if I am wrong…

  15. TamaraISchmitz
    March 18, 2013

    You understand the trade-offs correctly!

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