[sdiy] decoupling caps again

[Ullrich Peter]

Hi!

I had also troubles in some design that missed bypass caps, but also no troubles when there were none.

It depends heavily on the circuit function, the opamp speed/slew rate, the power concept (thin power wires 
with recognizeable length versus low impedance power planes).
In case of nice power planes for the supplies (so a well designed multilayer PCB)  the need for bypass caps 
near every part is not given.
It doesn't disturb but the low impedant power supply can deliver enough energy in most cases.

You can see this in combination of the EMC part. The German Professor Niels Dirks makes some nice seminars 
That are focused on the PCB design regarding EMC. He tells that in the optimum case one capacitor group consisting of
About 3 to 6 capacitors - selected to have their minimum impedance points spread over the spectrum up to 2 GHz
Deliver a very low impedant power supply.
In general this works quite well, but due to components that need vias for their routing you have no ideal PCB so
The practical solution for us was to make 4 to 6 component groups with their own capacitor group.

Once I got a nice filter module out of an AKG studio digital delay. I also got the test report from the Precision One test 
site so I knew the module was working properly. It was an active opamp filter module (10 pole, 22kHz corner frequency)
and I wanted to make a simple test with a function generator on the test board. But I didn't receive a nice result. No audio
could be heard.
When I attached the scope I saw there there was an oscillation with 600kHz with rail to rail level. As 600kHz was far
Above the hearing range I couldn't hear it.
Looking closer at the circuit I saw that they had no bypass caps near the opamps on the module. Adding the bypass caps 
on the supply pins of the module made them work as fine as I expected and the oscillation disappeared....

Ciao
Peter

-----Ursprüngliche Nachricht-----
Von: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] Im Auftrag von Ian Fritz
Gesendet: Montag, 2. September 2013 23:55
An: Ingo Debus; synth-diy diy
Betreff: Re: [sdiy] decoupling caps again

I agree with the first comment below the article.  I'm sure that most diy-ers have at least one decoupling cap where power enters the board.  That already makes our typical boards different from the author's experiment, where he was comparing to the case where there was no decoupling at all.

Just because the author saw some oscillations in one particular circuit/layout doesn't mean you have to put decoupling caps on every chip.  The earlier studies, in fact, showed that a few caps here and there on an analog board are plenty.  This has always worked well for me.  In fact, I've never found a problem of any kind that could be fixed by adding more bypass caps.

Ian


At 01:25 PM 9/2/2013, Ingo Debus wrote:
>Hi all,
>
>in the past, we often discussed decoupling caps. Several people said, 
>in audio circuit these are not that important, since there are only 
>fairly low frequencies involved. Have a look at this:
>
><http://e2e.ti.com/blogs_/b/precisiondesignshub/archive/2013/08/13/the-
>decoupling-capacitor-is-it-really-necessary.aspx?HQS=hpa_prechubdecoup_
>130901&DCMP=mytinwsltr_08_31_2013&sp_rid_pod3=LTIyNjA0OTg5MDgS1&sp_mid_
>pod3=4968551>
>
>Ingo
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>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl
>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

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