2
\$\begingroup\$

Why does this 5V circuit use two of the EXACT same value capacitors?

It's not needing a larger capacitance issue considering 100nF is relatively small.

Here is the full schematic: Arduino Leonardo

Arduino 5V filter? capacitors

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Could you post the whole schematic? What kind if application is it? What range of frequencies? \$\endgroup\$
    – Leo
    Jun 27 '15 at 3:01
  • \$\begingroup\$ I'll second @Leo. Sam, please post the entire schematic (or a link to it), or at least a larger portion (subsystem) of the schematic. Without the bigger picture this question is unclear. \$\endgroup\$ Jun 27 '15 at 4:50
  • \$\begingroup\$ Link to full schematic added. \$\endgroup\$
    – Samee87
    Jun 27 '15 at 5:18
  • 4
    \$\begingroup\$ There's a very informative (and extensive, as usual with Dave Jones) video that closely relates to the topic: EEVblog #742 – Why Electrolytic Capacitors Are Connected In Parallel \$\endgroup\$
    – n.st
    Jun 27 '15 at 6:52
7
\$\begingroup\$

Because they're decoupling caps for two different ICs or supply paths. On the schematic they're side-by-side, but on the actual board they're over by the ICs they're decoupling.

\$\endgroup\$
4
  • 2
    \$\begingroup\$ Or two pins of the same IC \$\endgroup\$
    – The Photon
    Jun 27 '15 at 2:27
  • 1
    \$\begingroup\$ I looked at the pcb file and you're right. Both on opposite sides of the microcontroller and relatively close where the Vcc pins are. \$\endgroup\$
    – Samee87
    Jun 27 '15 at 3:28
  • 1
    \$\begingroup\$ And it's cheaper to have only one value in stock... \$\endgroup\$
    – jcoppens
    Jun 27 '15 at 3:48
  • 1
    \$\begingroup\$ @Samee87 I would add that it is not best-practice to place decoupling caps in the schematic like this. If possible they should be placed near the chip they are decoupling, so as to give away their function at first glance. Otherwise, imagine the horror when looking at the schematic of some very complex circuit with dozens of chips (glue logic, MCUs, FPGAs, each of which could need several caps): dozens of ~100nF caps directly across the rails in the same place in the schematic! \$\endgroup\$ Jun 27 '15 at 13:02
4
\$\begingroup\$

When you're using a decoupling capacitor like that you're trying to provide a low impedance path for transient current demands by your IC. The effectiveness of this is going to be directly related to the impedance of your decoupling capacitors, their mounting, vias, and "electrical" distance from your IC (the load that's demanding the current). By that I'm really talking about the inductance of the cap and all that stuff I just mentioned between it and your chips power pins.

This graph illustrates it nicely, capacitance dominates the impedance at lower frequencies until you get to the resonant point where inductance takes over. enter image description here

Using two caps in parallel will give you a lower impedance path (think two resistors in parallel), allowing you to either draw more dynamic current, or giving you less ripple in your voltage when you do. Although you are still bound by the package impedance of the IC.

\$\endgroup\$
2
  • \$\begingroup\$ Thanks for that answer. So at higher frequencies, the circuit looks more like an inductor which limits the ability of the power supply to source more current when needed? \$\endgroup\$
    – Samee87
    Jun 27 '15 at 6:43
  • \$\begingroup\$ Yes but I would say that the inductance limits the ability of the capacitor to source current when needed. Your actual power supply is likely too far away electrically to supply current at high frequency. That's one reason for using power planes instead of long thin traces, the impedance between the supply and the load will be lower. Also even if it was really close your power supply can't regulate fast enough for high frequency current demands, which is just another reason to use capacitors. \$\endgroup\$ Jun 27 '15 at 15:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.