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Ground Planes? - RF Cafe Forums

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RFGuest

Post subject: Ground Planes? Posted: Mon Jan 23, 2006 5:15 pm

Hi all! I was reading about ground planes and it seems like the majority of the advice points to a design where two ground planes are created for analog and digital signals and only connect them at one point near the power supply. But I was reading some more articles and noticed, this:

"A number of different ground plane strategies can be adopted for a radio product PCB. There is no unique answer to the best strategy for a given type of product. Some people are great advocates of split ground planes for analogue, digital, radio or audio circuits. Experience on a number of radio products at Plextek has shown that a single low impedance ground plane for all parts of the circuit is usually a good starting point. Often attempts to split the ground planes causes more problems than it solves. Careful consideration of the flow of currents throughout the product is essential to minimise digital interference with audio and radio circuits. Given the proliferation in the use of DSP and microprocessors in radio products this is a very significant issue."

Can anyone comment from experience? Thanks!

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Guest

Post subject: Ground planesPosted: Tue Jan 24, 2006 3:30 pm

The major issue is how much non-DC current flows in the ground region.

I've had good results both ways, but there are a couple of non-obvious (to most people) things to watch out for:

1. Ground loops. If using separate ground planes, proper ("star") grounding becomes more significant. Ground loops can be hard to avoid anyway, but can be critical for small-signal applications requiring low noise pickup.

2. Proper bypassing. This reduces the difference between the two configurations. There need to be both small, high-self-resonant-frequency capacitors at the point of power use, the power plane/ground plane capacitance, and a bulk capacitor as well. There's a lot of material about this on the web - unfortunately, not all of it is correct. Watch out for unwanted resonances.

Basically, keep an eye on where the current is flowing.

Good Luck!

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IR

Post subject: Posted: Wed Jan 25, 2006 3:05 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello guest,

The issue that you raised is a very acute one. I had the same concerns on my last PCB layout, which was for a large Mixed-Signal board with close to 2000 components and included high-speed Digital sections and RF sections up to 400MHz. What I did is using separate ground planes for the Analogue/RF sections and digital sections and added optional resistors at the schematic to connect between these planes. I connected via holes from the different GND planes to the top layer and making an option for connection between the planes through these resistors if neccessary.

Eventually I connected between the planes through the resistors. Indeed, there is no one-way solution to this issue as the 2nd guest mentioned in his reply. Therefore, you should make an option in your layout to cover both options.

Good luck!

_________________

Best regards,

- IR

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RFGuest

Post subject: Posted: Wed Jan 25, 2006 5:37 pm

Thanks for the info!

I've seen that term star grounding before. Is that similar to starpoint ground? If so, can someone kindly explain this? I am not familiar. Thanks!

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IR

Post subject: Posted: Wed Jan 25, 2006 5:57 pm

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello RFGUest,

Here is a link for you with relevant information:

http://www.aikenamps.com/StarGround.html

Additional information can be found by Google!

_________________

Best regards,

- IR

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itsfrigid

Post subject: Posted: Sat Apr 01, 2006 5:13 pm

Lieutenant

Joined: Thu Mar 23, 2006 4:18 am

Posts: 2

you might wanna have a look at the high speed PCB layout seminar by analog devices -

http://www.analog.com/en/content/0%2C28 ... %2C00.html

scroll down for this seminar.

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