IC Decoupling

[patroclus]

I'm building quite a complex circuit, and it has three 74HC595 (serial to parallel). Well, I managed to make them work fine, after lots of problems (I had to add pull resistors in clock inputs, etc..) but when I add decoupling capacitors (between VCC IC pin and groung) it starts to get wrong outputs.

I use 0,1 uF ceramic capacitors for the task.
Isn0t it suppose to wirk better with decoupling capacitors?? It works great for me without them, and bad with them... why?...

 

[Russlk]

You have to analyze where the high current is flowing and put the decoupling caps where they will provide the shortest path. If you get them in the wrong place they just couple noise from one circuit to another and cause problems.

 

[patroclus]

So, I have to put them just next to VCC pin, but how do I know there's a big current? I mean, next to VCC pins are the power lines (I'm building it in a protoboard, and controll it by parallel port, and supply 5V from external adaptor + 7805 + filtering capacitor + diode).

Now I completed the circuit, adding a few more ICs, and decoupling capacitors help now a LOT. If I don't put them in, all goes wrong, and if I put them in, it gets better (but not fine yet... )

any help?