Decoupling tutorial with visuals

TAMeyer

Member
Greetings:

I found this of worth, perhaps others will too.

Dangerous Prototypes - A decoupling tutorial with supporting screenshots using a scope. Helps explain the “why”.

DP's introduction (click on the words "excellent tutorial" in line two).
http://dangerousprototypes.com/2011/11/02/decoupling-capacitors-explained/

The article itself.
http://www.vagrearg.org/?p=decoupling


Detail:

The forum has discussed decoupling at length. Senior members have provided excellent tutorials and explanations. They have been read, but for me the concepts didn’t sink in. It had nothing to do with the material and everything to do with my stage in the learning process. Google searches also found many articles but nothing "stuck".

D/P just awarded Bertho the Grand Prize for this years 7400 contest.
http://dangerousprototypes.com/2011/11/02/open-7400-competition-winners/

His tute's introduction reminded me of the car story below.
The screenshots were helpful for a non-engineer like myself.

Bertho’s literary continuity is noted too. A 7400 series chip is the test subject.



Backgrounder:

Growing up, my father gave me the basic tools – wrenches and knowledge - to take care of an auto. He reviewed the information when I bought my 1st car. Sensing the impatience of an 17 year old male, he knew the lectures could only be a certain length.

It was about a year later when, as the dealership mechanic put it, I had “soaked” the engine’s rings by overfilling the crankcase during an oil change. After the spelling lesson ($oaked), the how and why of an engine became more important. Of more interest too. I started buying my own tools. Remember timing lights?

More as an obligation than anything else, decoupling caps have been included in my designs for sometime. Annoyed with board real estate issues, last year I called a friend who is an EE asking "Why?". He replied with a question about water towers. It was then that I began to understand.

Seeing the scope images made the concept stick.



Thanks

Terry
 
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inglewoodpete

Senior Member
If you're worried about real estate on circuit boards, look at using surface mount decoupling capacitors. The picture is of a 28-pin PICAXE but the 40-pin chip can be decoupled in much the same way. You can a pretty good job on the 8, 14 and 20-pin PICAXEs too.

BypassCap1a.jpg

You'll have to click on the picture to be able to see the capacitor properly.
 
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Dippy

Moderator
I could see 4 pictures of 'scope displays but where was the explanation and example circuit?
Am I meant to click another button?
All the ones I clicked led me round in circles... sorry to sound thick but where do I click to get the details?
 

Dippy

Moderator
Ah, you have more patience than me Andrew, thank you.
A nice article.
It's a pity it'll go the same way as the decoupling App Notes I have posted (at least) twice.... :(
 

TAMeyer

Member

Dicky Mint

Senior Member
Practical and theoretical reasons not to skimp on the supply decoupling! An interesting article and discussion. As ballpark figures are we looking at ~100nF per PICAXE chip? Or more? Sorry I got a bit lost with the calculus!
 

eclectic

Moderator
Practical and theoretical reasons not to skimp on the supply decoupling! An interesting article and discussion. As ballpark figures are we looking at ~100nF per PICAXE chip? Or more? Sorry I got a bit lost with the calculus!
The 100 nF is the "standard" capacitor included on all
the Picaxe boards.

e
 

Dippy

Moderator
As usual, it depends on the app and the environment. Good old Horses for Courses.

100nF should be fine Dicky in general, people usually choose 22nF to 100nF, but what kind of resistor would you choose?
And when you have a slightly wobbly supply it is often decoupled with a combo of little one plus //d with an electro.

Many people (all pukka designers) consider decoupling as 'standard practice' especially in switching circuits and ADC work. In RF/HF work you often parallel several caps due to their characteristics.

Most people forget (or don't know) that micros are, in addition to being affected by noise, also noise generators.
Take a look at the dsPIC family data sheet for more info.
The more sensitive your circuit then the more careful you have to be.
The trouble is that some people throw various caps in all over the place without thought.
Size, type and position are all extremely important.
 

Dippy

Moderator
Cheers Westy. I'd probably want to do a rewrite anyway with someone reviewing it privately first.

Then Technical can have a new "How To" Dropdown in the Forum....;)
 
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