Decoupling Capacitors

I have a question about the decoupling capacitors across the +12v, 0 and -12v rails on Eurorack modules. This isn’t specific to Mutable Instruments modules really, but module design in general.

The question is very simple: how do you decide what capacitors to use for this?

Looking at lots of modules I see a variety of different values used here. My understanding is you need some smaller capacitors (i.e. 100nF) and some larger ones (i.e. 47uF) to filter out different frequencies of power supply ripple, but even with Mutable Instruments modules there seems to be a bit of variety in the exact values (i.e. some modules have a lot of small 100nF capacitors in parallel).

Are these values determined by some calculation? Or do you start with something that seems sensible and then tweak it over time based on issues that come up (for example with the MCU not starting)? If you were designing a new module now would you just use the same arrangement of decoupling caps from your last module or would it need tweaking depending no the module?

Also, the larger ones are mostly electrolytic capacitors. Is this just due to price or is there a reason electrolytic capacitors are better in this situation?

The last question is what would happen if you got the values of these capacitors wrong? Would it lead to problems with the modules starting up, or maybe noise appearing on the output?

I’ve searched for this quite a few times but I only ever find vague rules of thumb and a lot of people seem to just guess or copy what everyone else does!

If you look at the board layout, you’ll notice that all these capacitors are located close to each IC. They provide a local source of charge when the IC needs it the most - for example during transients. If you see 10x 100nF caps in parallel, the goal is not to reach a value of 1µF, but to actually have 10 little reservoirs for each IC.

No – I tend to use 22µF for smaller modules (power consumption < 50mA) and 47µF above that threshold. 10µF or less for low-HP utility modules.

Electrolytics aren’t great – they have a limited life (especially if you stress them by applying a voltage close to their maximum rating) and they are bulky. But they are indeed less expensive than ceramic capacitors (and less prone to shortages, especially during the great capacitor draught of 2018-2019).

The most likely scenario is noise creeping into an analog circuit. That said, I’ve never been in a situation when I had to deal with a noise problem that could get fixed by more bypass capacitors.

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Thanks for the comprehensive response - you’ve answered all of my questions!

I’ve basically been doing something very similar, putting a 100n ceramic and 10u electrolytic on each rail and decoupling caps near the ICs as their datasheets recommend.

And even just prototyping and doing very small batches of PCBs has been a nightmare with the shortages. I don’t even want to know what it’s been like trying to do a proper production run!

It seems like some stuff is starting to come back in stock though.