Electronics > Beginners
Are de-coupling caps always needed?
T3sl4co1l:
Two capacitors in series are equivalent to one capacitor of, essentially, the same overall component length.
If that length is adequate for the amp's stability, it won't matter if it's delivered from one up close, or one at a modest distance (lead length), or two (or more) in series (having the same overall lead+component length).
Tim
Wimberleytech:
--- Quote from: David Hess on December 20, 2019, 09:21:12 pm ---
It is more complicated than that because decoupling depends on the current return path from the output which can go to either the positive or negative supply. Analog devices published an excellent application note included below which shows the details of what is going on.
--- End quote ---
Excellent article. The key takeaway is that you need to see what the manufacturer recommends unless you have the skill to peer inside the architecture and figure it out on your own. Quite often, though, the details of what is inside is not published, so again, follow the recommendations of the manufacturer.
Jan Audio:
When i started i thought the same : i already have a cap at the PSU so its good.
Then i found out : your ICs can also make the power line dirty, fast logic can give noise back to the power line.
From now on i have all ICs a 100n ceramic cap, and no ceramic at the power supply, only a big elko.
By example a optocoupler i place the 100n not close to the IC, instead i make sure the optocoupler-resistor is also behind it, as close as possible ofcourse.
David Hess:
--- Quote from: Wimberleytech on December 22, 2019, 03:23:15 pm ---
--- Quote from: David Hess on December 20, 2019, 09:21:12 pm ---
It is more complicated than that because decoupling depends on the current return path from the output which can go to either the positive or negative supply. Analog devices published an excellent application note included below which shows the details of what is going on.
--- End quote ---
Excellent article. The key takeaway is that you need to see what the manufacturer recommends unless you have the skill to peer inside the architecture and figure it out on your own. Quite often, though, the details of what is inside is not published, so again, follow the recommendations of the manufacturer.
--- End quote ---
The situation is not quite that dire. Almost all outputs are either uncommitted to one power rail (open collector/drain/emitter) (1) or totem-pole to both. The designer gets to choose whether the load terminates into ground or one of the supply rails so can know exactly what the current return path is.
(1) I have never seen an open source output but ECL is open emitter. The LM311 comparator can be configured as open collector or open emitter.
exe:
Some opamps can oscillate internally, like ne5532, or many hi-speed opamps.
As of "signal frequency", there is enough EMI around...
One can try to get away with old and slow opamps such as tl072 or lm358 that work well on a breadboard with long wires and without decoupling, but even then I'd add decoupling for better signal integrity.
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