Electronics > Projects, Designs, and Technical Stuff

power decoupling myths

<< < (13/13)

T3sl4co1l:
Again, for what?  This is not a cut-and-dry situation!  If you need simple answers, just bypass it and be done; if you find problems, spin the board with more filtering.

Tim

David Hess:

--- Quote from: exe on July 25, 2020, 10:37:58 am ---Can you advice me on which ferite beads to use? From what I read, there is always a danger of resonance between ferite beads and decoupling caps. Would it help to use the beads with the biggest dc resistance? (I bought some with 2 Ohm beads, but never tried them in action). I work with low-power stuff (dac, adc, mcu) and my concern is noise injection into analog circuitry.
--- End quote ---

I have never heard of that happening with ferrite beads because they are designed to be very lossy, but I have had it happen with low loss inductors which would have been more suited for a switching power supply.  The solution to that is a parallel resistor across the inductor to lower the Q and this decoupling configuration is common in old test equipment between assemblies.

At a low enough power, RC decoupling becomes reasonable.  A ferrite bead normally only provides a couple hundred ohms so if the current draw is low enough for a couple hundred ohm or larger resistor, just use the resistor.

exe:

--- Quote from: T3sl4co1l on July 25, 2020, 03:14:38 pm ---If you need simple answers, just bypass it and be done; if you find problems, spin the board with more filtering.

--- End quote ---

Making new boards is what I'm trying to avoid :). My current approach is to add a footprint for the bead. If it's not needed I'll just put a zero-ohm resistor.

Siwastaja:

--- Quote from: exe on July 26, 2020, 08:27:37 am ---
--- Quote from: T3sl4co1l on July 25, 2020, 03:14:38 pm ---If you need simple answers, just bypass it and be done; if you find problems, spin the board with more filtering.

--- End quote ---

Making new boards is what I'm trying to avoid :). My current approach is to add a footprint for the bead. If it's not needed I'll just put a zero-ohm resistor.

--- End quote ---

Note that if you are doing one-offs, say, less than 5pcs, it's likely a more optimum configuration to skip that footprint altogether and just bodge when necessary. Optimum in terms of the big picture, that is.

This is because 95% of the time, you don't find any problem and don't need to add the bead, so you end up soldering those jumper resistors by default. All of this is extra work in schematic, layout, and hand-fabrication you need to do every time, for every unit.

Instead, I tend to design in what I expect to be the final configuration, then if I missed something or measure an unexpected problem, I bodge it to the PCB, cutting a trace and adding a ferrite bead somewhere is not too difficult if it only happens 5% of the time.

Generally, I have been reducing the "trying to think about every possibility beforehand" mentality, reducing placeholders, extra prototyping features etc., because I found out I rarely have time to try and measure every possible thing I thought would be nice-to-have beforehand. And when I do have a problem, it happens to be something I did not expect to see... So I try to make prototypes as close as the final product as possible, then work around the limitations when necesssary.

Number of cases I have needed to re-spin a prototype PCB (something serious enough that it can't be bodged): 0.

But this is just my €0.02.

electrolust:
https://www.picotest.com/products_Decoupling_Test_Board_Kit.html#gsc.tab=0

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod