Decoupling caps value

[VEGETA]

so 0402 10uF cap can work as good as 100nF 0402 (nearly) due to the package size. I just wanted to know the go-to value in normal situations.

Yes, but note that in $current_year, 0402 10uF is not realistic; if some manufacturer sells such a thing you should expect a significant drop in capacitance (maybe even -90% down to 1uF) under any DC bias, even at just 3.3V. If your task is to filter at 3.3V, then excess capacitance at 0V does nothing useful but increase the initial inrush energy or make hotplug ringing transients more difficult to deal with. So don't go overboard. 1µF in 0402 is a good go-to MLCC instead of the classic 100nF. If you don't work in a field where certification of every component is required, like most of us don't, being able to substitute different values helps tremendously given the dire situation with availability. Maybe 0.56uF is suddenly available for cheaper? Good for you!

sorry for my mistake but i meant 1uF 0402 not 10uF.

thus we can safely say that anything between 100nF to 1uF 0402 X7R is a good value and case size to pick in general. Plus adding some bigger elec. cap to filter ripple and dampen the rail.

[Siwastaja]

Yes, I can totally agree with your conclusion and this is what I do.

Also remember that while the tabulated package inductance difference is small between 0402 and 0603, 0402 allows you to do much better layout when you have a package with 0.5mm pitch so that you need to place dozens of parts (power bypass caps, ADC RC input filter caps, output pin series termination resistors) around it. 0201 would be optimal but is a tad difficult to work in manual prototyping and some not-up-to-date fabs might have problems with it. But 0402 works with manageable amount of breaking-out in layout while 0603 requires spanning the parts significantly further away.

[MisterHeadache]

Wes Hayward provides an explanation of the interactions between multiple filter cap values on his web site.  Scroll down about half-way where he shows how the  self-resonances of multiple caps can interact and create essentially nulls in the filtering frequency response.

http://w7zoi.net/bypass/decouple.html

[Siwastaja]

Wes Hayward provides an explanation of the interactions between multiple filter cap values on his web site.  Scroll down about half-way where he shows how the  self-resonances of multiple caps can interact and create essentially nulls in the filtering frequency response.

http://w7zoi.net/bypass/decouple.html

Great resource, one point I want to bring out to add to my earlier posts, this guy says it better:

There is some virtue to this resonance.   There are some applications where it is desirable to get a very good bypass or good decoupling at one specific frequency.   However, in most cases, we want good wide band performance.

(emphasis mine)

[David Hess]

5- having elec. cap to dampen the rail is always recommended. having a relatively big elec. cap (10~22uF) as a local area storage is good too...won't enhance decoupling much but will help in ripple and stuff like that.

The electrolytic bulk decoupling capacitor is often located at the power input and perhaps at the end of the distribution network on the board.  The best model I have found for it is that its ESR terminates the impedance of the power distribution network absorbing any reflections, which agrees closely with the rule of thumb of 50 microfarads per amp.  Another way to think of it is that the bulk decoupling capacitor quashes the Q of the transmission line forming the power distribution network.

100 nF was always "I don't know, lets put something" value anyway. In most cases, it is fine, especially on devices with a lot of VDD pins. If you find out that in some design it is not enough, you can fine tune it.

Ignoring EMC, see below, one way to calculate the minimum decoupling value is by how much charge the IC is switching.  The ratio of charge stored in the coupling capacitor and the load determines how much voltage ripple will be produced.  I found that this gives very accurate agreement with measurements.

Regarding 100nF, it's really the "bare minimum" value. Given DC bias, temperature, tolerance and aging, let's say it goes down to 50nF. Then, at 10MHz, which is a relevant figure because it's tested for EMC and it's well within the edge rate capability of microcontroller / logic IC IO, impedance is
1/(2*pi*10e6*50e-9) = 0.32 ohms,
which means a switching current peak of 1A would cause 0.3V voltage drop due to used charge from that capacitor, which then the upstream power trace would then try to supply, limited by its inductance, causing a dip in the supply voltage and then opposite overvoltage peak (ringing). Now this doesn't seem too bad but redo the calculation with 10nF and you start seeing why I suggest the classic 100nF is the "bare minimum" which works in most cases fine but does not have much margin. Therefore, I second the recommendation you often hear from EMC experts, use 1uF as your default bypass cap if you can get it in the same package size you would use with 100nF.

I remember one design where decreasing the decoupling values reduced EMC because the series inductance combined with the lower capacitance produced a null at the frequency of operation.

[Siwastaja]

I remember one design where decreasing the decoupling values reduced EMC because the series inductance combined with the lower capacitance produced a null at the frequency of operation.

The fact people sometimes see mild successes by decreasing C also keeps feeding the myth of smaller C having any advantage. You can see this in my above post where I posted two links; both show a very narrow region right at SRF where the impedance is lower than that of the largest capacitor. So yes, the smaller cap gives better HF performance but only at that very narrow frequency band.

The problem is the narrowness of this dip, and it being dependent on unit-to-unit variation, DC bias, temperature and ageing of the capacitor. Therefore, if such change to smaller capacitance value makes the EMC pass, the result is not valid as it depends on that exact part found at the lab. Lab of course writes the report for you, and for self-certified stuff, if you don't understand what happened, then no one cares even if your product is in reality non-compliant, until someone does independent testing.

If you look my second link, you can kinda see that if you wanted a modest say 3-5dB improvement over wide band, you would need to parallel not 3 but 10-15. maybe 20 different capacitor values! This is obviously impossible because you could not physically fit them close enough. So aiming for SRF dip is not going to make it; instead the correct way for the improved high-frequency attenuation is lower the ESL i.e., use a smaller part and/or better layout, or paralleling multiple caps (the same part number!) if layout enables this.

Ignoring EMC, see below, one way to calculate the minimum decoupling value is by how much charge the IC is switching.  The ratio of charge stored in the coupling capacitor and the load determines how much voltage ripple will be produced.  I found that this gives very accurate agreement with measurements.

Excellent and simple way of thinking, I like it. I regularly use this approach when choosing power bypass cap or bootstrap cap for gate driver ICs, knowing the Qg_tot of the MOSFET. Make the capacitor 100x or so. Same thing when using simple RC filter to provide low impedance for SAR ADCs for those slowly moving inputs you won't want to spend an opamp for; use 1000x the internal sample&hold capacitor (again a simple datasheet value) for roughly 10-bit performance.

[David Hess]

I remember one design where decreasing the decoupling values reduced EMC because the series inductance combined with the lower capacitance produced a null at the frequency of operation.

The fact people sometimes see mild successes by decreasing C also keeps feeding the myth of smaller C having any advantage. You can see this in my above post where I posted two links; both show a very narrow region right at SRF where the impedance is lower than that of the largest capacitor. So yes, the smaller cap gives better HF performance but only at that very narrow frequency band.

The problem is the narrowness of this dip, and it being dependent on unit-to-unit variation, DC bias, temperature and ageing of the capacitor. Therefore, if such change to smaller capacitance value makes the EMC pass, the result is not valid as it depends on that exact part found at the lab. Lab of course writes the report for you, and for self-certified stuff, if you don't understand what happened, then no one cares even if your product is in reality non-compliant, until someone does independent testing.

If you look my second link, you can kinda see that if you wanted a modest say 3-5dB improvement over wide band, you would need to parallel not 3 but 10-15. maybe 20 different capacitor values! This is obviously impossible because you could not physically fit them close enough. So aiming for SRF dip is not going to make it; instead the correct way for the improved high-frequency attenuation is lower the ESL i.e., use a smaller part and/or better layout, or paralleling multiple caps (the same part number!) if layout enables this.

It was a while ago so the design was through hole and they were using stacked metal film capacitors with 5% tolerance and no adverse characteristics.  For a while back then stacked metal film capacitors were as inexpensive as ceramic capacitors.  I recognized them because I was also using them for decoupling applications.  They had a specific EMC problem and changing the capacitors values fixed it without a redesign.

[The Electrician]

Random image from Google image search:
https://www.doeeet.com/content/wp-content/uploads/2019/10/Impedance-of-usual-MLCCs-16V-rated-vs-DC-voltage-bias.png

Choose some Z value you deem sufficient, let's say 1E-1 ohms and say do you see any advantage in using the smaller capacitance part?

Another:
https://res.utmel.com/Images/UEditor/c8cd4daa-b2ee-42e6-80ba-59384db825b8.png

This would be worth reading: https://web.archive.org/web/20200518044500/https://www.ultracad.com/mentor/esr%20and%20bypass%20caps.pdf

[Smokey]

I thought this was interesting.  At least from a historical perspective of how we got the 0.1uF, 1uF, 10uF set.

https://www.signalintegrityjournal.com/articles/1589-the-myth-of-three-capacitor-values