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Questions relating to designing PCBs
By tonyget
#177847
I'd like to know: why in many PCB designs, there are several small capacitors placed in parallel instead of using one bigger capacitor ?

For instance, this "SparkFun FTDI Basic Breakout", can't you replace C2 and C5 with one 10.1uF capacitor ?


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By uChip
#177881
Smaller capacitors respond to higher frequencies better. Larger capacitors respond to lower frequencies better.** To get broader coverage sometimes you combine a smaller capacitor and a larger capacitor.

** Smaller or larger isn't just the capacitance value or the physical size. It is also the construction materials of the capacitor. However the best construction materials for high frequency response generally don't make the larger capacitance values so the correlation above serves. A tantalum capacitor is an example of good frequency response materials that also yield larger capacitance values, but they are also much more expensive. So again, using two cheap capacitors can be a better (for some definition of better) solution.

You will also see many small "bypass" capacitors scattered around the PCB. In this case it is not the aggregate capacitance that is important. Instead what is important is that the capacitance is really close to the portion of the circuit (typically an IC) that the capacitor is serving. Again, this is about frequency response, but here the critical factor is the length of the conductor between the capacitor and the IC. Keeping that conductor length short allows the capacitor to do its job supplying instantaneous current where it is needed and suppressing noise.
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By Ross Robotics
#177901
Yep, and you can also Google, "decoupling capacitors."
By tonyget
#177979
Thanks for reply.

I have googled "ESI" "ESR", learned some interesting characteristics capacitor exhibits at various frequency.