0
\$\begingroup\$

I'm using an MCP1700 low-dropout voltage regulator to power an XBee separately from my other Arduino circuit, because it can use up to 215mA all on its own.

There are 1uF decoupling caps on both the input & the output of the VR (as necessary according to the datasheet), but I also followed through with advice given by Digi to put 1uF & 8.2pF caps right on the Vin line of the Xbee.

I didn't notice at first since they are separated out a bit, but looking at my schematic, since there is nothing else getting power from the 250mA VR there are actually 3 capacitors between those two, the 8.2pF one and the two 1uF!

Will the 2 caps do a worse job than just the expected 1? They are parallel so combine to 2uF, is this ok?

Just wanted to double check before I consider desoldering one of them...

\$\endgroup\$
3
  • \$\begingroup\$ I can't imagine the 8pF cap is adding anything useful to the mix. The datasheet specifies a ceramic 1µF capacitor. If you deviate from ceramic and change it for an electrolytic cap, you should possibly add a small 100nF ceramic in parallel. 8pF is about 3 or 4 orders magnitude too small to really make a difference. \$\endgroup\$
    – jippie
    Aug 15 '15 at 15:27
  • 1
    \$\begingroup\$ Since the Xbee is operating at either 900 MHz or 2.4 GHz, the 1uF capacitor is probably quite high impedance at that frequency - see i.stack.imgur.com/znd6q.gif and hence not a very good decoupler. The 8.2pF cap is there to stop the RF getting propagated onto the power rail. \$\endgroup\$
    – stefandz
    Aug 15 '15 at 15:36
  • \$\begingroup\$ OK, sounds reasonable. @stefandz \$\endgroup\$
    – jippie
    Aug 16 '15 at 11:18
3
\$\begingroup\$

This all depends on the frequency of current draw of the device on the line, the frequency response of the capacitors used, their ESR, parasitic inductance of traces joining capacitors and loads and actual physical layout. The purpose of decoupling capacitors is to provide a local power source for the load and keep the voltage rail clean (these two are different ways of saying the same thing, effectively).

Placing individual capacitors physically close to the IC they are supposed to be decoupling minimises trace inductance and hence maximises their suitability for purpose - so the fact that you have three capacitors all on the same line adding up to ~2uF doesn't mean that you could replace the three with one 2uF capacitor and have it act exactly the same - each is connected to the next via a small resistor and a small inductor representing the impedance of the copper between the two.

How much ripple can you tolerate on the power rails? If you know this you can measure existing ripple under worst-case conditions, remove a capacitor of your choice and repeat the measurement. If that takes you out of specification - put it back! Don't forget that you will need to repeat this measurement at each of the ICs you have with the shortest possible ground lead for the 'scope. You should be able to get hold of some ground springs like the one shown below to help this. If you use the standard lead you get with the scope, you will be measuring with its inductance in place too which will make your measurements pretty much meaningless.

Oscilloscope probe ground spring

So what should you do? Well, if it were me I would leave the existing caps in. They cost almost nothing and aren't likely harming anything. If you really need to remove one or all, then do it using the approach I have described.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks for the detailed answer. Yeah, can't forget the additional resistance of the wires. I'm definitely leaving them in, it's the extra work of taking them out I was trying to avoid. \$\endgroup\$
    – Glebbb
    Aug 18 '15 at 5:35

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.