This. Just had an eye opening session in LTspice yesterday after getting the numbers for parasitics of the caps put in. Sure that 100p cap will have a nice low impedance dip at high frequency, but it's gonna throw a peak in at a lower frequency. If its amplitude is enough to matter depends on the rest of the curve.
Yup!
This is more or less the 2nd order equivalent model from Dave's video (counting order as 0 = just capacitors, 1 = capacitors with inductors between them, 2 = capacitors modeled with ESR+ESL, 3 = including dissipation and loss, coupling between inductances, etc.):
What Dave misses is that impedances are complex valued, so when putting two together, one above and one below SRF, the two components are actually acting as an inductor and capacitor and therefore you get a parallel resonance.
Adding trace inductances isn't necessary to show the effect, but it's a more realistic result.
Instead, if we use just two capacitors, one of which has ESR added (or we use a lossy type like a tantalum*), the result is much better:
*Polymer and tantalum caps are available in all sorts of ESRs, from very low to very high. Most polymers cluster at lower ESRs, while most tantalums cluster around a fractional ohm; but whichever type you're shopping for, understand that the ESR is a critical component parameter, and you have your choice of it from either family. And if you don't like either, you can always add a resistor to a ceramic -- no shame in that, though it does increase the ESL somewhat.
Instead of having a worst-case impedance of 2 ohms at 20MHz, this has a worst-case impedance of 1 ohms (or 0dB) at low frequencies, getting better as frequency rises, until ESL takes over.
Huh, the scale changed, that's weird. The asymptote is also higher: it passes 2 ohms at about 50MHz, instead of 200MHz. This is effectively L2 + L6, don't know why I had made L6 larger in the latter case. That perhaps represents a somewhat lazy condition, where there's a few more mm of trace length between 0.1uF cap and load. Rest assured, for a comparable layout (i.e., distance between pin and bypass cap, same trace width), the asymptote is the same -- it's just a straight proportion of length.
Tim