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Old 1st March 2004, 06:39 PM   #11
Oli is offline Oli  United Kingdom
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Default Let me get this clear...

Let me get this clear....

With the VHC logic...

I can use 220u OS-CON and 0.1u ceramic and use ferrite beads to decouple the supply.

Q. Will the initial inrush current 'blow up' my 0.5A rated Cho-Drops?


With the OPA627 op-amp...

I can decouple with 220u OS-CON and 0.1u ceramic and place 10R resistors in the supply lines.

Q. Do I strictly need 2.5W resistors, or is the pulse length so short that a smaller power will do e.g. 0.125W?

I too have seen a lot of talk about low impedance supplies for audio op-amps. Is this simply a myth? If we add resistors do we not encourage a less robust supply?
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Old 1st March 2004, 07:19 PM   #12
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Default Re: Thanks... Anyone tried 'Cho-Drops'

Quote:
Originally posted by Oli
Thanks folks!

Indeed a very good technical article Elso. I need to spend some time to digest at 80 pages.

I am currently using the following devices with your asynchronous reclocker:

Cho-Drop

The data sheet claims they are better than ferrites.

Has anyone else had any experience of these?
interesting devices (or at least their name), but impedance wise they are no better than ferrites. They claim to be flat or reasonably flat between 50 MHz and 200 MHz.

What would be the advantage ? In series with signals I can see advantage, in supply lines I just want the highest impedance in the range of interest.

And they claim ferrites resonate, but they do not explain why the cho things wouldn't resonate

Anyhow, interesting
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Old 1st March 2004, 07:41 PM   #13
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Default Re: Thanks... Anyone tried 'Cho-Drops'

Quote:
Originally posted by Oli
Thanks folks!

Indeed a very good technical article Elso. I need to spend some time to digest at 80 pages.

I am currently using the following devices with your asynchronous reclocker:

Cho-Drop

The data sheet claims they are better than ferrites.

Has anyone else had any experience of these?
Just browsed. It is a not so good article. Wrt part 1

these guys still splitt ground planes, probably never heard of Kirchoff. And they believe quiet digital signals exist. Mr Fourier, where are you ?


Part 2 is OK as far as I can see
Part 3 is OK (Walt Jung was involved)
Part 4 is interesting (Thermal management)
Part 5: Measures OK, rationale lacking
Part 6 (shielding): Nice examples

After all it is not so bad but

NEVER split ground planes

Ciao
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Old 1st March 2004, 08:16 PM   #14
Oli is offline Oli  United Kingdom
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Default Thanks for reading

Thanks for reading it Guido.

Have you any thoughts about my previous message - about the resistor power rating?

I notice that in one of your articles you avoid bypassing Os-Cons with other capacitors due to further possible resonances. Is this a good idea? I have 100Mhz signals- surely Os-Cons alone will not do the bypass trick...
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Old 1st March 2004, 09:20 PM   #15
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Default Split Ground Planes

Guido,

We always used multiple ground planes to reduce the noise is or Seismic Systems. So are you saying that incrorrect! Multiple ground planes are one of the best ways to reduce circulating ground currents.

So if that you mean split grounds are worse that a single ground plane for mixed signals application your wrong. Or maybe I have missunderstood you.

I had poor luck with HC types of logic. Best to stick with ALS or LS. CMOS can cause the the ground to bounce as it charges it up going from a high to a low state. Of course having multiple ground reduces this problem.
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Old 1st March 2004, 09:39 PM   #16
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Default Re: Re: Thanks... Anyone tried 'Cho-Drops'

Quote:
Originally posted by Guido Tent


NEVER split ground planes

Guido,

I am building a DAC. The DAC chip itself has analog and digital grounds. My original plan (and I have started layout) is/was to split the analog plane and connect them together at a point of my choosing (with an impedance of my choosing).

I totally want to understand your grounding proposal, but reading your posts did not make it click for me.

Would you care to elaborate so I can understand how to best take advantage of your advice?

Current setup is double sided (DAC chips both sides) SMD devices using 4 layer prints where I intended to have one power plane (split) and one ground plane (split but connected).

Thanks in advance.

Petter
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Old 1st March 2004, 11:23 PM   #17
Oli is offline Oli  United Kingdom
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Default A single ground plane!

To my horror I believe I am just understanding Guido's ground plane theory:


It's a single common analogue/digital ground plane!


Splitting the ground plane creates a 'bottleneck' for return currents to flow through and often a long current path. This is especially true if current needs flow from one side of the split to the other side.

A single large ground plane will reduce the aforementioned bottleneck and reduce the loop size for return currents.

As long as the plane covers sufficient area any delicate ground inputs should not be affected by large return currents, since resulting potential drops will be minimal.

Guido, do I correctly understand it?

Does this mean all the grounds on my current design: PLL, DGND, AGND etc should form one continuous ground plane? e.g. Solder AGND and DGND pins of my CS8412 to the common ground plane?

Interesting stuff... I am having to rethink my ideas about that stuff we call electricity...

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Old 1st March 2004, 11:31 PM   #18
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Default Ferrites............

Yes, there are lots of different mixes, but usually only 3 get made into the little beads that we use, and one of them is by far the most common.

That is the one that Guido refers to.

Jim:

I think that in your old application, splitting the planes worked better. But for this high speed stuff, the loop is too large, and the EMI goes up.

So...........it depends on the application. I guess.

Jocko
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Old 1st March 2004, 11:44 PM   #19
Oli is offline Oli  United Kingdom
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Default Simpler design

If my understanding is correct...

On the bright side...

A single ground plane is easier to implement!


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Old 2nd March 2004, 12:04 AM   #20
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Jocko,

These are the practices we used at TI, and the stuff you can read in application notes from Analog Device, Burr Brown and others. If you building true RF hardware that is way out of the spectrum of the low frequencies, we see in CD and DAC's. Of course, some oscillations caused by poor ground, cheap diodes logic that's to fast create RF in low frequency designs.

Having said that, it is possible with do a careful layout to use a signal ground, however impedances and across the plane must be kept very low along with the supplies. Also, it possible to add extra grounds on buss bars, this can reduce the cost, I've done that before.

As we discussed, on RF shield must be connected on both end but on audio circuit require a connection a source. Doing this reduces currents in the shield.

Right, it depend on application and money and skill.

Keep the loop tight
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