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I'm build a synthesizer (up to 2GHz) and got a questions regarding the gound planes (analog vs. digital).

According to the datasheet (p. 7) some pins must be connected to analog GND, ex. pin 10 must be connected to the analog GND plane.

http://www.analog.com/static/imported-files/data_sheets/ADF4351.pdf

I read somewhere that it is best to have separate ground planes, connected at a single point. But somewhere else I read that it is best to have one gnd plane, and just be carefully with the the placement of analog and digital blocks.

But what is the best pcb layout design?

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    \$\begingroup\$ If you're asking questions like this, you're not ready to make stuff that has to work at 2 Ghz. \$\endgroup\$ – Connor Wolf Mar 21 '13 at 8:05
  • \$\begingroup\$ The correct answer is "it depends". What else (major chips/functions) is in your design? What is the form-factor? How are you doing your 3D shielding? etc... \$\endgroup\$ – DrFriedParts Mar 21 '13 at 8:17
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    \$\begingroup\$ @Conner Wolf, Yeah maybe my question was a bit odd formulated, but what I meant is: what are your favored method? I think I will go for a single point connection (star topology) with separate planes, to ensure digital return current does not flow though analog ground region. \$\endgroup\$ – JakobJ Mar 21 '13 at 12:18
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At 2 GHz I strongly recommend a single ground plane.

Separating planes with a star topology certainly has its place in low noise audio designs, keeping digital noise out of analog circuitry.

But at 2GHz, the dangers of accidentally building resonant circuitry and antennae when you divide up the ground plane completely outweighs any possible benefits.

Use a single ground plane. Keep all loop areas as small as possible. Minimise inductances; if necessary, use multiple vias in parallel around decoupling capacitors. Decouple as close as you can to PSU and ground pins.

Or invest in serious signal integrity simulation tools (Hyperlynx?) and get/create Ibis models for everything, spend a few months learning the tools and simulating the exact PCB layout until it's right.

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In fairly simple terms....

Analogue ground planes carry analogue currents to and from various parts of the analogue circuit and reduces AC differences between circuits as much as it can. However, it's not 100% effective and small AC volt-drops can develop but, they do not tend to have a severe detrimental effect because they are either additive or subtractive to the signals. As frequencies rise this problem can get worse and more rigorous lay-out techniques are required.

Digital ground planes carry digital currents and these currents tend to be pulses and are usually not related to analogue currents because, typically, they may be bus currents or currents associated with your wide-band frequency synthesizer's control port. If these currents shared the same ground plane as the analogue ground plane they would be destructive to the purity of the analogue signal and cause noise problems and reduce the performance of the circuit.

Joining them at one point avoids the digital currents trying to circulate through analogue parts of the ground plane.

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