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Old 2nd March 2004, 12:10 AM   #21
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Default I GOT TO HAVE A SIGNAL POINT GROUND

Oli,

What you can do is to add jumpers to connect the ground together. Also, add extra hole for power so you’re lay out is flexible.

So when you find that your noise is to high you can disconnect the ground planes. Or if you find the noise is ok you can leave the jumpers in. This also allows to pick several different location on the PCB where you'd thinks the ground currents will cancel.
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Old 2nd March 2004, 01:14 AM   #22
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Default Grounding Methods

Grounding Methods
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Old 2nd March 2004, 12:13 PM   #23
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Default Re: A single ground plane!

Quote:
Originally posted by Oli
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...

Oli,

Yes, you understand it right. In the case of ADC or DAC chips: Think of the loop the currents run that have to pass the A/D or D/A barier. Induced voltages in that loop may work out quity nasty/.

The bottom line

- All gnd pins of all parts to 1 plane
- Reduce currents through plane by
- Reducing current amplitude and frequency content
- Reducing the common part of currents (i.e. place decoupling caps - aka decaps VERY close to the ground pins of the ICs)
- Place ICs interacting with RF signals close to each other (use common sense)

sp the plane will be near equi potential

succes
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Old 2nd March 2004, 12:43 PM   #24
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Hi,

I think there is a misunderstanding regarding split ground planes. If one reads Analog Devices app note more carefully they do advocate puting complete ADC, DAC and clock source all on the same analog ground plane. You would use split plane only if digital part of the circuit is something like Pentium computer.

I have another question. I use SMD parts and double sided board. Because almost all of the connections are on the top side, top ground plane has many holes. Bottom ground plane is almost unbroken. Is there any advantage by stitching bottom and top ground plane with vias at regular interval (100-200mils) as oposed to placing vias only when there is discontinuity?

Second since board is fairly small, I can use thinner 0.8mm vs standard 1.6mm PCB. Would there be any advantage in that?

Best regards,

Jaka Racman
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Old 2nd March 2004, 02:07 PM   #25
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If you have to use one ground plane, it should be continuous. Havening the plane on the bottom connected via feed through is ok, but the plane has to be a continuous plane.

Still the optimum way of connecting mixed signal with precision converters is to use multiple continuous ground planes.
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Old 2nd March 2004, 02:17 PM   #26
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Default Grounding methods one

Grounding methods one Single Point, Parrallel and Mulripoint Grounds.
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Old 2nd March 2004, 02:18 PM   #27
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Default Grounding Methods 2

Single point ground
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Old 2nd March 2004, 02:19 PM   #28
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Default Grounding Methods 3

Multipoint Grounding.
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Old 2nd March 2004, 02:22 PM   #29
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Default Grounding Methods 4

Pratical Grounding Methods
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Old 2nd March 2004, 02:27 PM   #30
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Default Grounding Methods 5

Low Frequency Grounding below 10 Mhz
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