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The importance of de-coupling.

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Kibi:
De-coupling is often "a potential trap for young players".
As some of you may know, I got my new 'scope the other day and as I was fiddling around with it, I thought I'd put this little demonstration together.
I have built a simple bog standard astable multivibrator using a 555 timer, oscillating at around just 6.5 KHz. In one instance I have omitted the 100nF de-coupling capacitor across the chip's supply lines and in the other I have included it.

Without de-coupling.





Notice the nasty overshoot and oscillations.


With de-coupling.





Almost no overshoot, and just a 500ns dip before recovering to the peak of the square wave.

At the cost of just a 100nF ceramic capacitor located close to the supply lines of the device, a lot of trouble can be avoided.
It doesn't matter how good you think you power supply is, you MUST employ de-coupling in your circuits. This was a mere 6.5KHz signal, imagine what would happen in the MHz range. Of course, you wouldn't rely on a 555 timer as your clock source in the MHz range, but this is just a demonstration of what can happen.

IanB:
Mike "Electric Stuff" has a neat demo of this on YouTube:

Kibi:
Thanks Ian / Mike for the very practical example. This just re-enforces the importance. A "young player" without a 'scope may not be able to understand why his / her circuit is not behaving according to design. The practical youtube example coupled (excuse the pun) with the 'scope readings should explain the importance of de-coupling.
If Mike had de-coupled each device with a capacitor close to it's supply lines then he'd have achieved a result much closer to the design.

ejeffrey:
Yeah.  You know how Dave always says the first rule of troubleshooting is 'thou shalt measure voltages'? If you have a scope, put away your DMM and the first thing you should do is measure the supply voltages with a scope probe.  You get a 2-for-1 checking both the DC voltage and the ripple waveform.

Kibi:
Ye, he is correct - again... :) No sort of DMM is going to detect an almost 2v overshoot of 2v for 100ns above the supply lines. If you were using classic TTL devices a 2v overshoot on the 5v supply lines or indeed a 2v overshoot (in my example) lasting a mere 100ns to their inputs is enough to destroy them over a short period of time.

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