Why do datasheets show multiple decoupling caps in parallel? Why not add them?

Discussion in 'General Electronics Chat' started by s_mack, Jun 6, 2015.

  1. s_mack

    Thread Starter Member

    Dec 17, 2011
    I've seen this a few times and I want to know if I'm missing something.

    Please take this example datasheet.

    In their typical application diagrams, they show pin 6 (VCC) having both a 0.1μF and a 1.0μF cap in parallel. The last paragraph on page 10 says, "For higher frequency transients, spikes, or digital noises on the line, a good low equivalent-series-resistance (ESR) ceramic capacitor, typically 1μF, placed as close as possible to the device VCC pin works best."... nothing about the 0.1μF. So now I'm left to wonder... should I be using 1.0μF, a single 1.1μF*, or both a 1.0μF and 0.1μF as shown?

    Is it carelessly written or am I missing something?


    * granted, in this case if 1.1μF is really required, I would expect to have to use both since 1.1μF is an uncommon (if non-existent) value cap.
  2. Brainbox


    Nov 15, 2010
    Did You read this sticky article?
    And for what it is worth:
    If You buy a 1 uF 16V ceramic capacitor class Y5U and You supply it with 12V, you probably are left with only 0.25 uF.
    So it makes no sense to searh for a 1.1 uF capacitor, unless You are swimming in Your money.
  3. s_mack

    Thread Starter Member

    Dec 17, 2011
    Well, no I didn't read that sticky because the title states it is defining why we need bypass caps... I know why we need bypass caps. I wanted to know why datasheets often show more caps, usually in parallel, and never with an explanation. In that sticky, it describes why we might need a mica cap in parallel with an electrolytic, and that makes sense if that were the case, but this datasheet/diagram says nothing about the properties of the 1.0μF cap except to say it should be low ESR which, if anything, suggests it is not intended to be electrolytic. Am I just to presume it is electrolytic, despite all lack of evidence?

    I acknowledged the futility of looking for a 1.1μF in the last sentence/footnote of my message. I was just using the numbers provided in the example datasheet.
  4. bertus


    Apr 5, 2008

    Did you read this PDF I posted in that thread?

  5. MrChips


    Oct 2, 2009
    It is not carelessly written. You are missing something. You cannot simply add the capacitances in parallel because they have different frequency responses.

    Read this:

    and this:
  6. crutschow


    Mar 14, 2008
    If capacitors were perfect then you would only need one.
    But higher value (typically electrolytic types) have higher ESR and inductance than smaller ceramic capacitors so it's common to put a higher value electrolytic which is better at filtering the lower frequencies in parallel with a small ceramic to better filter the higher frequencies.
    Also since lead and trace inductance can reduce the effectiveness of decoupling capacitors at higher frequencies you often find one large electrolytic somewhere in the circuit along with a ceramic capacitor directly connected to the power and ground pins of each IC .
  7. ramancini8


    Jul 18, 2012
    The applications engineer who wrote the data sheet likely used 1UF electrolytic or tantalum and .1UF ceramic or mica, so he recommended those values. More important is the question, "how do they work in your application"? Try different values and observe the results; soon you will find out that many values work, but dielectrics are critical. Once you establish good values and dielectrics you will select capacitors based on size and price, and you will use the selected parts for almost all designs.