I use some decoupling capacitors (100 nF) for low speed ICs (< 1 MHz) and I need some recommendations to verify my routing. My layout uses two layers:
- Top (Red) with Vcc
- Bottom (Green) with GND
And I have this IC as an example:
The Vcc layer is connected to my cap and the cap is connected to the IC over pin 28. So the IC is directly connected with the cap and not with the layer.
Here is the question:
Should I do the same for the ground connection or is it fine to connect the cap as shown in the image? The "bad" thing with DIP-packages is the long routing distance between GND and Vcc (pin 14 and pin 28) for example.
Then the next question is how do I handle decoupling capactitors for ICs like the FT232RL which has more GND than Vcc pin. Should I connect all GNDs and route them to the GND of the cap or can I connect them directly with GND.
What is a smart way to do it?
Update: Add the changes
Best Answer
Considering the small green traces leaving pins 15 to 21 at the south side, you probably have completely cut the ground plane connection to pin 22 and pin 14.
It looks like the ground connection from the capacitor to pin 28 runs (way) south of pins 15 to 28, heading north, west of the IC. (Or it runs way east of IC). Anyway, with such a long trace, I doubt the capacitor functions as a decoupling capacitor.
I would recommend to delete all the Vcc polygons/plane, try to swap the mentioned bottom traces (originating from pins 15-21) to the top and apply a decent (uninterupted) ground plane in the bottom layer.
Only when necessary (when traces have to cross other traces), use a vias to swap the trace to the bottom side, but keep those bottom traces as short as possible.