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Old 6th February 2012, 11:56 PM   #351
gootee is offline gootee  United States
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Quote:
Originally Posted by gorgon53 View Post
Gotee, glad you pointed to the importance of high enough capacitor value, the lower the voltage and the higher the current the bigger the values needed. As at the same time you also want the lowest ESR and ESL possible it is really a challenge to find suitable types
Yes, and inductance of the connections for the decoupling capacitors can be a killer, especially as the caps get larger.

I am really starting to think that, sometimes, maybe even often, the only way to achieve a low-enough inductance for decoupling (and a low-enough impedance in general), as seen by a power output stage active load, and ALSO achieve a practical (i.e. implementable) physical placement and connection scheme for the decoupling caps, will be to use multiple smaller parallel caps, each with their own separate parallel connections to the points of decoupling.

But, at that point, maybe we should just "go all the way" and use multiple parallel conductors for each power and ground rail, too, everywhere after the first smoothing cap in the PSU.

That's all assuming the use of typical DIY construction techniques, i.e. a PCB with one or two sides with copper, or point-to-point wiring. But I STILL also think that it would be quite simple to use multiple pieces of PCB and fabricate a mutlilayer board assembly by hand, at home, even though it would require some workarounds, like large access holes to be able to solder on an inner layer, and vias would have to be hand-made, i.e. with a short wire inserted and soldered on both ends, et al.

Multiple layers would certainly make layout MUCH easier, especially because ground and power planes could be available basically everywhere. (And if we started with extremely-thin PCB laminates, we might even get some decent amount of high-frequency capacitance between layers.)

I still keep thinking back to the LM3886 chipamp's decoupling capacitor placement and connection problems. The pin-out basically just sucks, for that, especially for a one- or two-sided PCB. Pardon my Francais.

Here: http://www.ti.com/lit/ds/symlink/lm3886.pdf

Last edited by gootee; 6th February 2012 at 11:59 PM.
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Old 7th February 2012, 07:52 AM   #352
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just n idea...
suppose we had a thin multilayer-cap of comparable high capacitance with the mechanical length of an ic and a width of an ic. The cap would be connectible on both sides along the full length of it. We now could stuck this cap directly under the ic and get the shortest possible connection. Ic manufacturers could even manufacture and supplie a ready made "sandwich" like this.
What do you thinck? Would possible resulting strayfields pose problems that could not be tackled?
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Old 7th February 2012, 09:40 AM   #353
AndrewT is online now AndrewT  Scotland
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There was a post a few years back showing this.
It linked to a pdf where the decoupling was explained and further it showed that the route of the decoupling leads across the IC pins should follow the internal current route.
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Old 7th February 2012, 11:37 AM   #354
marce is offline marce  United Kingdom
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You can get those sort of decouplers for PTH DIP components, but when you move to SMD, that where the problems arise. The added problem is through hole components are in decline especially IC's, and SMD while it can be done on two layers, it is always a compromise.

As for two layer boards, they use to be equal 50-50 with multilayer, the split for 2010 for worldwide PCB production was 60% multilayer 11% two layer, the rest was made up of flexis, flexi-rigid, IC substrates and CEM.
And another amazing fact 6 billion passive components are placed on PCB's every day...

Last edited by marce; 7th February 2012 at 11:47 AM.
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Old 7th February 2012, 11:48 AM   #355
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hi marce, never came across such devices, could you point me to a source pleas?
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Old 7th February 2012, 12:06 PM   #356
marce is offline marce  United Kingdom
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I will try and find a source, I havn't used them since about 1990! so I may not have much luck. They were very flat and the pins slotted in with the IC pins, when good old 0.3" wide logic ruled the day.
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Old 7th February 2012, 12:31 PM   #357
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thnx, they migth be quite useful sometimes
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Old 7th February 2012, 02:29 PM   #358
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hi Andrew, can you please give me a hint how to find this pdf?
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Old 7th February 2012, 02:31 PM   #359
AndrewT is online now AndrewT  Scotland
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I'll see if it's in my documents on this HDD.

I've set it searching for *decouple*.pdf
This could take a while !

It's still searching.
Has found "decouple" in 70 pdfs so far.

This could take a long while.
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Last edited by AndrewT; 7th February 2012 at 02:55 PM.
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Old 7th February 2012, 03:42 PM   #360
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thanx
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