Do chips really need multiple values of decoupling capacitor in the same packages?
This is a question I've been wondering my self from time to time, and I haven't found an answer yet. I did a simulation with LTSpice to get some kind of answer. I chose a couple of capacitors from Murata pretty much on random: 4.7 µF https://psearch.en.murata.com/capacitor/product/GRM155R61A475MEAA%23.html and 100nF https://psearch.en.murata.com/capacitor/product/GRM152B31A104KE19%23.html

I set the ESL for both caps to 300p and ESR for 100 nF to 30m and for 4.7 µF to 8m. With these values their impedance seems to match quite well to that in the Murata's graphs. (To be precise the ESL is not exactly the same, but it is close enough so I'll use the same value)

I simulated with only 4.7 µF, 4.7 µF + 100 nF and 2 x 4.7µF. I added 1 nH inductance between the capacitors, to simulate the trace connecting them.



The results are interesting, but not very unexcepected

Adding the 100 nF increases filtering, except for the antiresonance frequency. Adding another 4.7 µF has the same effect, except that there is no antiresonance. The 100 nF works better at its self resonant frequency, but it's effect is smaller than the lost filtering performance of the antiresonance. Based on this, I'd just add more bigger capacitors.

But, if you e.g. had a noise problem at 30 MHz, then it makes sense adding that 100 nF capacitor, because it does filter that frequency well.


  Q1: Is that 470pF capacitor really helping?


At it's resonant frequency it is. If there is no noise at that frequency, then not that much.


  Q2: Wouldn't it make sense to replace all three of them with a single 1uF capacitor in an 0201 package?


Would be probably better to add two 1 µF 0201 capacitors. Then if you do run in to trouble at some certain frequency, you could change one of them to capacitor that has SRF at that frequency. You could also leave the other one as not assembled, but capacitors are cheap so why bother.


  Q3: When people say that a higher value capacitor is less useful at higher frequencies, how much of that is due to the capacitance, and how much is due to the increased package size usually associated with larger caps?


Pretty much is about the package size. Of course the higher SRF helps again, but only if you have noise at that frequency. Otherwise it is just better to double the biggest capacitance.