Schematic notes: Place capacitor as "close" to pins: Is this a necessary note? Is it a significant point?

Question

Background

I am building a ATMega4809 board that allows the board to be programmed using the Arduino IDE. While examining the following schematic I noticed that where the caps are placed the author included a note on each.

Here's a close up of the note:

Question: Close To Pins?

My question is, "Is this a mis-understanding by the author, or is it actually necessary that the capacitor be placed very "close" to the pins which they are attempting to shield?

Close seems a bit subjective. Should you solder them to the pins? Should they be in the breadboard holes exactly next to the pins. Do you just insure that their is nothing (no other component) "between" the cap and the pins?

Does this really matter -- have significance?

In contrast, I can imagine if they were "far" there could be a difference possibly. But if they're all connected it seems as if it wouldn't matter.

I've (attempted) to use capacitors like this in the past and I'm wondering if I should consider this as I use them in the future.

Answer 1

Capacitors don't "shield."

Capacitors are placed on the power supply pins to absorb noise coming in from the power supply and to supply short bursts of high current when the processor switches states.

Long wires act as inductors. That reduces the effectiveness of the capacitors.

1. The author really meant for you to place the capacitors as stated in the comments in the schematic.
2. "Close" means as close as phyically possible with the smallest inductance conenctions possible. Short, wide traces between the capacitor and the pins. Ideally, you'd use a surface mount capacitor and snuggle it right up beside the pins.
3. This not an ideal circuit for use on a wireless breadboard - getting the oscillator to run correctly will be painful.
4. Capacitors placed just anywhere because they are all connected together is a bad plan. Put each capacitor where the schematic tells you.

Answer 2

The point is to avoid excessive inductance between the capacitor and the chip it's decoupling. If you have long PCB traces between them, that's an impedance that prevents the capacitor from supplying current as quickly as it could otherwise, limiting how fast of a transient it can respond to. In turn, that limits the maximum speed the chip it's decoupling can run at, whether that be frequency in a CPU or output pulse characteristics in a gate driver, or whatever else you're using.

Remember that wires aren't just wires, they always have parasitic elements involved. At high speeds, the inductance and capacitance of PCB traces mandates careful layout design, and that's what these notes are intended to help with--they're telling you that this is a part of the circuit that's particularly sensitive to layout. In extreme cases, manufacturers will even give you complete layout recommendations, showing exactly where you should position capacitors and resistors and copper traces to ensure their chip works as specified.

Answer 3

It matters. They should be placed as close as possible to the pins while maintaining all the design rules of the PCB fab and assembly. They should all have good low-impedance connections to ground as well.

This is to keep the parasitic inductance (and even resistance) to a minimum. These "bypass" or "decoupling" caps provide a local energy reservoir for high current transients. If these transients had to come from far away, the series inductance and resistance would cause a dip and ringing in the supply, which can cause erratic operation or failure to operate at all.