Electronics > Beginners

Are de-coupling caps always needed?

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David Hess:
It depends on the parts involved and the loads and the circuit layout.  Slow parts, meaning low gain-bandwidth product parts, have low di/dt (slow change in current over time) so inductance in the supply connections has less of an effect and decoupling can be located further away, possibly at the power supply.  A circuit using 4 LM324s over 8 square inches could likely get by with just a 1 microfarad tantalum or 10 microfarad aluminum electrolytic capacitor across the supply pins our regulator's output.

JustMeHere:
What do decoupling capacitors do?  They provide a shorter path for your current to set up.

When you create a signal current, as it flows down you trace, a return current must be set up at the exact same time.  This return current is essentially the magnetic field.  So before your signal current can move down the trace, the magnetic field must be set up between your positive charge and ground.  The more distance the current moves through the longer it takes to set up.  Two ways to address this are to run your positive and gnd trace next to each other, or -- even better -- have a big copper plane under your trace.  The closer the return path and the signal path, the less time it takes for the magnetic field to set up.

When the term frequency is used, it does not always mean what you see in a sine or square wave.  It has to do with the edge time.  Picture a square wave that is switching between 0 and 1 at 1 MHz.  In order for the signal to get from 0 to 1 fast enough for it to happen 1 million times a second, the rate at which the voltage needs to change must be fast enough.  On your oscilloscope, your frequency counter will see the 1 MHz signal.  Now let's change our square wave into just a single transition from 0 to 1.  The rate of that transition will still be the same.  Now the signal doesn't transition back to 0, your oscilloscope will not see a frequency.  The transition did happen and it happened at the same speed of a 1 MHz signal.   Thus it is important to consider the speed of the step edge when you are thinking about frequency.

So what does the decoupling capacitor do?  It provides a very short path down to the ground plane.  We have heard the term "capacitors pass AC and block DC."  AC is changing current and DC is steady current.  The current from the magnetic field passes through the capacitor.  Since the capacitor is close to your IC, is small, and connects to the ground plane the magnetic field can do this much more quickly then if the field has to "reach" around the board to set up.  Since the magnetic field sets up more quickly, the decoupling capacitor allows your signal to change more quickly.  Inductance is reduce and we know high frequency currents take the path of less inductance.

Since the magnetic field sets up more quickly, it also had to spread out less in order to set up.  Most of the magnetic field is between the trace and the ground plane.   This "loop" to the gnd is very small.  If field has to set up across the board (because there is no close return path) then the magnetic field of the ground loop will set up between the trace and the ground point.  It will travel through everything in between.  So reducing the size of the loop by using the decoupling capacitor, you also reduce the size of the ground loops and the amount of magnetism radiated across the board.   This reduces noise.  So decoupling capacitors also reduce the noise that your changing current creates.

I've mentioned ground plane in this explanation.  If you don't have a ground plane and use traces to provide the gnd, then the shape of your traces is important.  If your grd trace follows your signal trace it acts much like the gnd plane.  The size of the capacitor (distance between the terminals) will be most of the distance between the traces.  If you route your gnd trace around the board, and away from the signal trace, then the distance between traces becomes more significant than the size of the capacitor.  This effectively removes the benefit of the decoupling capacitor.   So if you are not paying much attention to your return path, decoupling capacitors might not be needed.

This response is getting too long, so I will stop here.  I may not be 100% correct, but I believe I am close.  The important thing to understand is that the 100nF high frequency decoupling capacitor helps the magnetic field's establishment time and reduces the field's size, and this works if you keep you ground loops small.

SL4P:

--- Quote ---What do decoupling capacitors do?  They provide a shorter path for your current to set up.
--- End quote ---
Is that a SHORT circuit ?   ;)

TimFox:
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.

level6:

--- Quote from: TimFox on December 20, 2019, 02:11:18 pm ---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 ---

Wouldn't you want a cap on the positive supply also, in the case of dual rails?

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