Amp-op decoupling and unused XOR gates

[wcampagner]

Hello,

I read that digital IC's require a small ceramic decoupling capacitor across Vcc and Gnd pins... does Amp-Op IC's need this capacitor too??

I'm designing a digital switching circuit for my stompbox... i'm usign a CMOS XOR gates... there is 4 gates in the IC, but i'll use only 3... what shall i do with the inputs and output of the unused gate? Do i need to connect it  to ground or Vcc or none??

Thanks again,
Wagner.

[R.G.]

When modern logic switches state, the circuits need current - instantly! The speed of switching is so fast that even PCB traces are too long for the current to come from the main power supply. Electrical signals travel in copper at about half the speed of light, or about 1 nanosecond per foot. You can get garbled signals for nanoseconds at a time, which is plenty to throw off logic.

Even when you are combining logic and analog, the logic causes nanosecond long glitches on the power supply that can send the analog stuff into a tizzy.

The way around this is to give each chip its own "canteen" of electricity so it can get a drink locally without it having to come through the long PCB pipes/traces to the chip, messing things up on the way. That's what a decoupling cap does. It keeps the pulses local, and off the shared PCB power traces.

Opamps really need this too, but for other reasons. It keeps their low frequency current demands off the power supply rails and keeps them from interacting and possibly oscillating because of the interaction. We often get away with not putting such bypass chips at opamps, but it is just getting away with it. In big setups with lots of opamps you usually can't get away with it. Sometimes for little circuits you can get away with it for logic as well, but always be aware that you are in fact getting away with it, not that it's not needed.

On the inputs; for CMOS never, never, ever leave inputs floating. Always tie them to some logic state. Ground is normal, but any solid logic state will do. You can use ground, logic 1 (the power supply) or even the output of some other gate that's switching. They don't necessarily have to be held quiet, but they have to be held **somewhere** otherwise they'll float around and pick up RF signals and other junk out of the air, with generally bad results.

So yes, tie the unused inputs somewhere. Ground will be very appropriate.

[Transmogrifox]

For perfboard circuits, I have found it convenient to tie inputs together, and then the output of the adjacent logic gate to that until I have worked my way around to either ground, Vcc or one of the used gates.  Then I tie the input of the "train" of unused gates to the output of a used gate, or ground or Vcc, whichever happens to be easiest.

If you're doing a circuit board layout, it's probably just as easy and just as well to tie all unused inputs to ground.....


.....just to add to what RG said to get my own 2 cents in.  It mostly comes down to what can be done the most neatly on whatever layout you're using.