Re: [SI-LIST] : Testing board stackup

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From: Wang Xiao-yun (wangxiaoyun@iname.com)
Date: Fri May 11 2001 - 04:40:18 PDT


Hello, Diego:
   I would like to suggest you to add one more layer to make the PCB structure
symmetrical and provide return path for analog signals instead of embedding
them
into two digital ground layers.
   For a mixed signals design, the most importance task is to suppress the
interference between analog and digital circuitry to minimum. This can be
achieved
by partitioning the two circuitry parts, as well as the signals properly.
   You are mentioning testboard and I assume it needs not pass EMC validation.
For a test-only board I normally tend to not over-constrain the design. For
example,
only keep your most critical signals well terminated and impedance-matched.
As a
complementary, keep most of the digital signals accessible on the outer
layer so
that it is easy to do some modification on copper in case there is problem.
I tend to
leave all the analog signals on the outer layers instead of embedded.
   Regards.

At 17:46 2001-5-10 +0200, Diego.KOCH@st.com wrote:
>
> Hi all,
>
> I have the following stackup in a testing board form mixed-signal
> devices:
>
> digital signal
> digital ground
> analog signal
> digital ground
> digital signal
>
> The digital signals have a risetime of 500ps. The analog signals are
> slow, nearly DC, but of little amplitude. When digital signals are
> switching, I have some noise induced in the analog traces. I suspect
> this is due to induced magnetic fields into analog traces.
> Is it due to wrong layer stackup ? If yes, how can I improve it ?
>
> Thank you for everything
>
> Diego Koch
>
>
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--
Wang Xiao-yun
Shanghai, China
善守者藏于九地之下,善攻者动于九天之上,故能自保而全胜也。

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