Electronics > Beginners

Are de-coupling caps always needed?

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T3sl4co1l:

--- Quote from: Wimberleytech on December 20, 2019, 07:55:31 pm ---Perhaps you misunderstood my reason for voting the comment as winner.  That answer conveys the complexity of the problem in the fewest number of words so I felt it deserved accolades.

I was not being pejorative.

--- End quote ---

I didn't feel my response was calling out pejorativeness?  I agree, and basically my response stands as a far less optimal example. ;D

Tim

David Hess:

--- Quote from: TimFox on December 20, 2019, 02:11:18 pm ---Unless the manufacturer's data sheet specifies otherwise, the most important bypass cap on an op amp is between the positive and negative power pins, by as short a path as possible.  This is a nuisance, since they are usually on opposite ends of the part.  The next most important cap is between the power pin from which the output voltage is generated (usually the negative pin) and ground.  Of course, single-supply op amp circuits usually ground that pin directly.  If you give a feedback amplifier an excuse to oscillate, it will take it.
--- End quote ---

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.

JustMeHere:

--- Quote from: SL4P on December 20, 2019, 08:47:21 am ---
--- Quote ---What do decoupling capacitors do?  They provide a shorter path for your current to set up.
--- End quote ---
Is that a SHORT circuit ?   ;)

--- End quote ---

No.  Look up displacement current, and watch Dave's video on capacitors passing AC current.  And Yes (whoops), after watching the video to the end, you're right too.  As the capacitor is charging, it is a short circuit.

Jwillis:
I have a noob question about decoupling caps on op amps . I read somewhere that aside from one at each rail tied to ground it was suggested to put one from rail to rail as well right close to the op amp . Is that really necessary?
Another question is since I'm running the ones I have (OPA445) at +55V and -15V I can't find any ceramics for that voltage. Higher voltage polys would work just as well right?

David Hess:

--- Quote from: Jwillis on December 22, 2019, 05:16:07 am ---I have a noob question about decoupling caps on op amps . I read somewhere that aside from one at each rail tied to ground it was suggested to put one from rail to rail as well right close to the op amp . Is that really necessary?
--- End quote ---

The application note from Analog Devices that I included in my earlier post covers what is required and why.  For a dual supply, sometimes the third capacitor is used to further reduce impedance when the load is attached to a supply rail instead of ground.  A load might be connected this way to keep its return current out of a sensitive ground circuit.


--- Quote ---Another question is since I'm running the ones I have (OPA445) at +55V and -15V I can't find any ceramics for that voltage. Higher voltage polys would work just as well right?
--- End quote ---

100 volt ceramic capacitors are common but plastic film capacitors work fine also.

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