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My quesiton:

Is it better practice to put same-valued capacitors in parallel of capacitors of different values to decouple the high-frequency noise caused by digital ICs?

Background

Digital IC need a decoupling capacitor close to their supply pins to ensure a stable voltage during power transients and to deal with noise (mostly to prevent noise generated by the IC to affect neighboring circuitry). It seems sensible to place a bulk capacitor (say 10-100uF), to act as an energy reservoir, and several smaller capacitors to deal with higher frequencies. The reason to place several small capacitors instead of just one is to deal with their Equivalent Series Inductance (ESL), which in practice, causes them to behave like an LC circuit.

The effect of anti-resonance

Yet, here is where best design practices and electronic myth seem to get mixed up and confusing to me. Most electronic engineers I have met like placing several decoupling capacitors of different values in parallel (with the smaller capacitors closer to the IC). The logic behind it is that the each capacitor takes care of a different noise frequency as depicted in Figure 1.

Figure 1: Impedance over frequency of three different value capacitors in parallel (cyan) vs their individual contribution (brown, blue, red). Image taken from https://www.allaboutcircuits.com/technical-articles/clean-power-for-every-ic-part-2-choosing-and-using-your-bypass-capacitors/

Figure 1: Impedance over frequency of three different value capacitors in parallel (cyan) vs their individual contribution (brown, blue, red). Image taken from All About Circuits.

Note the small anti-resonance peak. It seams no major trouble , and the overall behavior of the three different capacitors in parallel is vastly superior to their individual decoupling capabilities.

However, I have read in Electromagnetic Compatibility Engineering by [Henry W. Ott] that placing capacitors of different values may cause a much greater antiresonance-peak which can be very harmful for our designs (see Figure 2). In fact, it amplifies any noise that falls into the anti-resoance frequency range, which is corroborated by this paper.

Figure 2: from Electromagnetic Compatibility Engineering, by Henry W. Ott, section 11.4.4

Figure 2: from Electromagnetic Compatibility Engineering, by Henry W. Ott, section 11.4.4. The 15nH inductance makes reference to the capacitors ESL.

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  • \$\begingroup\$ You should think of each real cap as an inductor, resistor and capacitor in series, then parallel a bunch of those up. The behaviour of the whole system depends on all three parameters and ideally needs to be tweaked until it is "good enough", which otoh depends on the requirements. "As flat as possible" thus is not a good requirement. \$\endgroup\$ – PlasmaHH Jul 25 '17 at 11:47
  • \$\begingroup\$ It depends on the whole system(containing power planes,decaps etc). When multiple numbers of same value capacitors are used there are high chances of Anti resonance with power planes, same way when multiple numbers of different values capacitors are used there are high chances of anti resonance b/w different capacitor values. Bottom point we need is simulate complete system with whatever tools we have(can be simple excel/advanced tools).... \$\endgroup\$ – user19579 Jul 25 '17 at 12:03
  • \$\begingroup\$ I trust Henry Ott. Also, most of the time I don't have room for multiple values on every pin. And as you move the capacitor away from the pin, its value for decoupling diminishes rapidly. You need to incorporate layout factors in your simulation. Trace inductance is not negligible. \$\endgroup\$ – mkeith Jul 25 '17 at 16:13
  • \$\begingroup\$ In many realworld cases, the resistances: of the capacitors, the wires, the PCB foil (500 micro_ohm/square), the solder!, provides adequate dampening. For 1,000uF and 100nanoH, resonating 15.9KHz, you need 0.01 ohms for Q=1. For 0.1uF and 100nanoH, resonating 1.59MHz, you need ONE Ohm for Q=1 dampening. \$\endgroup\$ – analogsystemsrf Jul 26 '17 at 17:01
  • \$\begingroup\$ Summary: for capacitors less than 1uF, you either pick lossy caps or lossy wiring (PCB foil, thin traces are lossy, but high inductance). And the temperature affects the loss of a capacitor. \$\endgroup\$ – analogsystemsrf Jul 26 '17 at 17:02
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I have a bunch of observations that I decided to make into an answer and please note that I'm quite happy to spend 30 minutes doing a simulation of this if someone can precisely state what the test circuit was that produced the large anti-resonant peaks.

Firstly, I'm not sure that I follow the precise circuit of what was described by Ott.

Are the 15 nH inductors in series with each capacitor as is stated? If they are then that is clearly wrong because the smaller capacitors will have smaller ESLs. Is there any mention of the resistive loading effect of the circuit the capacitors are "smoothing"?

What are the inductances of the traces that feed the capacitors or, were the capacitors connected using earth and power planes?

In short, I'm not happy with the Ott claim based on the lack of clear circuit that can be reproduced in a sim. If a clear circuit can be made available then I'm interested!

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  • \$\begingroup\$ Thank you very much for your response. As I understood Ott's chapter, he models the trace inductance and the ESL for each capacitor as a 15nH inductance. He assumes each capacitor is connected with its own trace, and I believe that he neglects the series resistance (of both the capacitor and the trace). Thus he simplifies things and models every capacitor as an ideal capacitor in series with said 15nH inductance. \$\endgroup\$ – andresgongora Jul 25 '17 at 13:06
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    \$\begingroup\$ @andresgongora That is my concern - he has modeled something that is unrepresentative of reality. \$\endgroup\$ – Andy aka Jul 25 '17 at 13:15
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Summary: the individual capacitors need dampening; for 100uF caps, the solder and PCB foil may suffice (10milliOhm, if L = 10nH); for 1uF, use 0.1 ohm; for 10nF, use 1 ohm, etc.

Here with 4 capacitors, 100U/1U/10n/100p and 10nH ESL, the peaking depends on the losses in each cap {I consider sqrt(L/C) a good start; thus 10nH and 10pF needs 3.1 ohms, which I have not used here; however, 10nH and 100uF need 10 milliohm, which is illustrated in the 3rd screenshot.}

Lets examine this response, with 1 microOhm ESR in each cap. Notice the lowest dip is to -120dB. Zsource is only 50 Ohms. enter image description here

Now this response, with 1 milliohm ESR in each cap. Zsource is 50 Ohms.

enter image description here

And now 10 milliohm ESR for each cap, Zsource is 50 Ohms.

enter image description here

And with 10milliOhm in each cap, with the Zsource now 1uH + 50 ohms

enter image description here

Here is (requested) SCE sim [available at robustcircuitdesign.com for free] with 4 identical 1UF caps, each with 10nH ESL and 10milliOhms. There are NO PEAKS, because the 10milliOhm dampens those peaks. [or is the Zsource, of 50 ohms and 1uH, what dampens??]

enter image description here

In prior sim, there was no peaking. So I insert 3 inductors between the 4 caps. Now a sim of those 4 caps + 3 inductors (PCB foil, 10nH each). Notice the peaking returns (the ESR is only 1 ONE milliohm, to show peaking), at -20dB.

enter image description here

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  • \$\begingroup\$ Thank you very much! Your simulations clearly show that, as stated by Ott, we can expect very high anti-resonance peaks at certain frequencies (here 1.05MHz, 10.5MHz, and 105MHz). Could you also simulate what happens when the capacitors are the same value to compare? With say 10mOhm ESR. I have no access to Signal Chain Explorer (sadly). \$\endgroup\$ – andresgongora Jul 26 '17 at 7:55
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    \$\begingroup\$ Signal Chain Explorer is free to download. \$\endgroup\$ – analogsystemsrf Jul 26 '17 at 16:24
  • \$\begingroup\$ Thank you for the reference. I've been playing with the software a bit myself. So, judging the experiments, unless I know exactly where my noise falls I'd rather want to use same-size capacitors, right? But because bulk capacitors have high ESL and ESR, they can be added into the mix without problems. Please correct me if I got this wrong. \$\endgroup\$ – andresgongora Jul 28 '17 at 11:23
  • \$\begingroup\$ I don't think using 1R dampening cap would make sense as it'd essentially negate the bypass functionality you want with the capacitor to start with. \$\endgroup\$ – Barleyman Sep 8 '17 at 14:18
  • \$\begingroup\$ @Barleyman I'm shying away from placing extra impedance on my application's power path. I've been running similar simulations to those of analogsystemsrf, and I'm seeing that antiresonance is more complex than expected. \$\endgroup\$ – andresgongora Oct 10 '17 at 7:51
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You're doing pretty good on getting to the parallel resonance already. It depends on your application. If you're trying to suppress/bypass for example ethernet peaks, you should use parallel caps which have impedance dips in the fundamental frequency and some of the harmonics.

The "perfect" solution is to use low-ESL type ceramics which are usually characterized by having the pads on the long ends. These tend to have impedance over the spectrum that's as low or lower than regular MLCC chips have in their dips. They're also less vulnerable to the impedance peaks because there's so little inductance involved.

Here's a good write-up of what's going on here, a major source of these resonances are component pads, power planes and vias, not so much the capacitor itself: http://ntuemc.tw/upload/file/20120419205619a4fcf.pdf

That looks like it'll go stale any time so I've mirrored it here: https://drive.google.com/file/d/0B7odw4GViWr4VVR6aVYyTkstZUk/view?usp=sharing

Some people think you shouldn't aim to get your dips on the fundamental switching frequencies anyways because it allows the chip to do faster edges but I'm not sure I buy that. The impedance dip would be on the fundamental frequency, not on the higher harmonics that makes that sharp edge.

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    \$\begingroup\$ Thank you for your fast response. My specific problem is that I want to decouple a circuit with lots of PWM in it. Meaning that I have no clear fundamental frequencies I can aim at. Should I keep the capacitors with different values though? \$\endgroup\$ – andresgongora Jul 25 '17 at 12:35
  • \$\begingroup\$ PWM is a bit tricky. Usually it's a pretty low frequency, no? So a large ceramic (10µF or something) should cover frequenc(ies) of interest. General rule of thumb with parallering caps is that you have a "large" tank capacitor (can be electrolytic or "large" MLCC) and then a smaller capacitor to catch high frequencies which can be tuned to fundamental switching frequency of the IC (think sampling frequency, PHY signaling frequency..). Unless you have a specific reason to catch specific frequencies like the square wave harmonics, leave paralleling different size MLCC caps well enough alone. \$\endgroup\$ – Barleyman Jul 25 '17 at 12:59
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    \$\begingroup\$ I think I have problems with the harmonics (very steep rises, which I can not afford to lose by damping them). Also Its high frequency (for PWM, that is) of up to 1 MHz depending on my system's configuration. EDIT: After your edit: thus, you recommend using same valued capacitors, right? \$\endgroup\$ – andresgongora Jul 25 '17 at 13:09
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To complement others' answers:

Same value caps can also resonate together if you consider they are connected together with non-zero inductance traces or planes. You most likely won't get a large resonance peak in the impedance, but you will get a bit of circulating current in the power/ground as the caps ring together.

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