Electronics > Beginners

Are de-coupling caps always needed?

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level6:
If I'm using an opamp for a very low frequency application, i.e. a voltage comparator, do I always need supply de-coupling capacitors at the opamp device? I already have de-coupling at the power supplies for the board I'm working on.

iMo:
Yes.

tggzzz:
You need to be an expert to predict when you don't need decoupling caps, and even experts don't get it right all the time.

The frequency of your signals is usually irrelevant; what matters is how your particular opamp in your particular circuit interacts with the inductance in the power leads and other "parasitic" capacitances. To make it simpler: if you use a 200MHz opamp for a 1Hz signal, then it may well oscillate at several tens of MHz :)

Old engineering adage: "amplifiers oscillate, oscillators won't".

AG6QR:
If you're using an op-amp as a comparator, the transitions from one state to the other may contain high frequency components, even if the transitions don't happen often.

There are multiple reasons why a comparator is better for the task than an op amp, but that's for another thread.

If there are truly no high frequency components in the input or output of the op amp, you may be able to do without the decoupling caps right near the chip.  But follow the data sheet, or deviate from it at your own risk.

ejeffrey:
Basically decoupling of low speed low density parts like most opamps and discrete logic is done by convention as it isn't usually worth the effort to figure out what you really need.  For high speed/density parts like big FPGAs or SOCs, or very high speed amplifiers you need to be more careful.

The part speed matters more than the signal.  A high speed opamp needs better bypassing even if the signal it sees is relatively slow.  It also depends on what else is in the circuit, how far away your "bulk" capacitors are, and how you are using the part, and what your requirements are.

Bypass capacitors are also important for EMI, so your circuit may work properly but radiate a lot of excess noise on the power supply wiring, which you may or may not care about.

So your circuit will probably work without local bypass.  Is it worth your effort to figure that out vs. just putting capacitor there?

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