noise filtering caps in the power rails

[gehan_s]

hi all,

I am routing this PCB in which I need to rout a couple of 6n137 digital opto isolators. Since they will be switching at about 1MHz I have added these 100nF ceramic disc caps in between each IC on the power rail. My question is, is it really necessary and what kind of caps, values are better for this purpose?

It would also be helpful if you can give me some general tips on PCB routing.

I have added a screen shot of the part I am talking about.



Thanks in advance.

 

[Inti Innovations]

Decoupling caps are really necessary. I'd add 1nF || 100nF at each of the ICs in your circuit. Choose capacitors with high ESR (low Q) to get wide decoupling bandwidth.

Routing tips, looking at your layout I would probably reduce the width of at least some traces. Also avoid relatively big copper planes around pins (pin 7,8) without thermal relief.
Generally I would avoid through hole technology..

 

[marce]

You havn't got much of an isolation gap, with traces running the way they are!
Decoupling is a whole subject in itsself, I would look around the web as there are hundreds of links on decoupling, also we have discussed it on this forum numerous times.
Decoupling is necessary with any circuit not just digital.

 

[vasaroopak]

A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit.
For example, if the voltage level for a device is fixed, changing power demands are manifested as changing current demand. The power supply must accommodate these variations in current draw with as little change as possible in the power supply voltage. When the current draw in a device changes, the power supply cannot respond to that change instantaneously. As a consequence, the voltage at the device changes for a brief period before the power supply responds. The voltage regulator adjusts the amount of current it is supplying to keep the output voltage constant but can only effectively maintain the output voltage for events at frequencies from DC to a few hundred kHz, depending on the regulator (some are effective at regulating in the low MHz). For transient events that occur at frequencies above this range, there is a time lag before the voltage regulator responds to the new current demand level.
This is where the decoupling capacitor comes in. The decoupling capacitor works as the device’s local energy storage. The capacitor cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current demands. The capacitors effectively maintain power-supply voltage at frequencies from hundreds of kHz to hundreds of MHz (in the milliseconds to nanoseconds range). Decoupling capacitors are not useful for events occurring above or below this range.

 

[Inti Innovations]

CUT Noise caused by other circuit elements is shunted through the capacitor. CUT

This is correct! The rest not so much.

 

[marce]

Actually Vasaroopak is quite correct, though rather verbose in his reply, the following paragraph perfectly sums up what a decoupling (or bypass) capacitor does.

This is where the decoupling capacitor comes in. The decoupling capacitor works as the device’s local energy storage. The capacitor cannot provide DC power because it stores only a small amount of energy but this energy can respond very quickly to changing current demands. The capacitors effectively maintain power-supply voltage at frequencies from hundreds of kHz to hundreds of MHz (in the milliseconds to nanoseconds range). Decoupling capacitors are not useful for events occurring above or below this range.

 

[keith1200rs]

Actually Vasaroopak is quite correct, though rather verbose in his reply

That's because it is simply a cut & paste from Wikipedia

https://en.m.wikipedia.org/wiki/Decoupling_capacitor

Keith

 

[Inti Innovations]

That's because it is simply a cut & paste from Wikipedia

https://en.m.wikipedia.org/wiki/Decoupling_capacitor

Keith

Only he copied the wrong part of the article. The intro is not exactly correct, the rest of the article is correct.