Optimal supply track layout for mixed signal chip on 2L PCB?

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rosvall
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Posted by rosvall: Thu. Apr 14, 2011 - 12:13 AM
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As my first SMD project i'd like to put together a DC-100MHz frequency generator. Oh yeah, it's also my first stab at high frequency PCB design. This should be fun... :roll:


Connect the same colors. On a double sided board.

It's a 400MSPS DDS chip in a LQFP48 package, that runs at 1.8V, 3.3V and 1.8V and needs digital and analog grounds.
I'd like to put it on a 2 layered board. And i'd like to get the cleanest possible output from it.

How am i supposed to properly power and ground the analog end (red and cyan), and where should i put the (smallest) bypass cap?
Considering the tiny-ass size of this thing, am i just worrying too much about this?
Any hints are appreciated.

Datasheet: http://www.analog.com/static/imp...

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Learning and Information, General Electronics
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GordonFreeman
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Posted by GordonFreeman: Thu. Apr 14, 2011 - 03:21 AM
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First of all, I don't think 100Mhz should be too big of an issue.

Quote:
I'd like to put it on a 2 layered board. And i'd like to get the cleanest possible output from it. How am i supposed to properly power and ground the analog end

Use a four layer board with two copper planes. The ground plane will do far more good than any trace manipulation will. Also don't make any right angles, use 45 degree maximum.

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ka7ehk
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Posted by ka7ehk: Thu. Apr 14, 2011 - 04:58 AM
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While daunting, its NOT that bad. Do not forget that you CAN run traces inside the pin square on the top side. At some point, the analog and digital grounds have to be connected together (typically at a single point). It may be possible to route both on the same layer. You do NOT need a single monolithic plane for each.

I think I would join all of the AVdd pins with a net of traces topside, inside the pin square. They can be routed below the center pad. And DVdd can also be routed topside, above the center pad.

Jim

 

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