Decoupling Capacitors

cblaircblair Forum Coordinator Posts: 0
Hi - I am working on a class project and read that decoupling capacitors are needed. Do I really need them? If so why?


  • rtkrtk Pune, IndiaPosts: 167Forum Coordinator


    Most of the boards designed these days are high speed boards with many IC's. These Ics will be subject to fast changing signals which require the power supply to retain a constant voltage or current source to these IC's. The fluctuation of the voltage or current in a power source is seen as noise in a circuit and as always we want to reduce the amount of noise.

  • cblaircblair Posts: 93Forum Coordinator
    What values of decoupling do I need and where do I place them?
  • VivekKVivekK Pune, IndiaPosts: 89Forum Coordinator
    The answer depends on several things. Is this for a single IC and power supply or a network of ICs used with a power distribution network (PDN)? For a single device, you can use the rule of thumb which is to use a decoupling capacitor about ten times the value of the decoupling capacitance. For the latter, you will need to consider the impedance of PDN and the charge required by your ICs. You will need to know if this is for a digital or analog application as the calculation of capacitance either depends on the rise time of the signal or the operating frequency of the IC. In either case, more decoupling capacitance is preferred. To more accurately address this problem use of a simulation tool to address any parasitic resonances that occur due to the board layout is needed. The decoupling capacitor is usually placed near the power supply and ground and as close to the IC as possible.
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