[SI-LIST] Re: digital to analogue noise coupling
- From: "Perry Qu" <perry.qu@xxxxxxxxxxx>
- To: "'steve weir'" <weirsi@xxxxxxxxxx>, "'si-list'" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 20 Jun 2005 09:47:41 -0400
Thanks, steve. The crosstalk is in the range of 60 mV. The return path of
the clock is a solid plane and the clock is routed on either side of the
plane. There are about total 12 drops on 2 branches of the clock (series
terminated at both branches). The stackup looks like:
Component (short analogue/digital breakout)
Sig2 (most of analogue routing)
Sig3 (clock)
Gnd
Sig4 (clock)
...
As I understood, there is some capacitive coupling as clock on sig3 pass
underneath component pads on component side. Also, some inductive coupling
is bound to happen as analogue routing on sig2 which is far away from the
plane will see noise induced from the clock loop (sig2/gnd) -- this is small
as most of the analogue traces on sig2 are routed perpendicular to sig3, and
there is only a few routed at an 45 degree angle. However, even with all
these capacitive/inductive coupling, it still does not explain why I see the
copy of the clock rather than glitch corresponding to rising/falling edges
only.
Regards
Perry
=======================================
Perry Qu
Design & Qualification, Alcatel Canada
600 March Road, Ottawa ON, K2K 2E6
DID: 613-7846720 Fax: 613-5993642
Email: perry.qu@xxxxxxxxxxx
=======================================
-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx]
Sent: Monday, June 20, 2005 9:03 AM
To: Perry.Qu@xxxxxxxxxxx; si-list
Subject: Re: [SI-LIST] digital to analogue noise coupling
Perry, how much crosstalk are we talking about?
A couple of things to consider: Planes don't provide effective magnetic
shielding to low frequency interference. Your amplifier nodes are likely a
pretty high impedance making a net TC well above the 244ns half period of
your CODEC clock. What does the return path for that clock look like? How
much drop in the path does the clock induce? And is that the image that you
are seeing?
Steve.
At 08:45 AM 6/20/2005 -0400, Perry Qu wrote:
>Good morning,
>
>I was asked to help troubleshoot a digital-to-analogue noise coupling
>issue on a design. There is a 2MHz clock routed underneath an analogue
>circuitry and we picked up a copy of the 2 MHz clock with much smaller
>amplitude -- small for digital but large enough to interfere with analogue
circuit.
>
>The exact same copy of analogue circuitry with no clocks routed
>underneath works fine.
>
>Checked the layout of the mal-functioning circuitry, I found that there
>is small capacitive coupling -- only happened where the clock routed
>underneath some analogue components pads; Inductive coupling is also
>small as most of the analogue circuit are routed either perpendicular
>or at an angle to the digital traces.
>
>The most puzzling part is that the crosstalk noise repeat the source clock.
>This seems to suggest that this is neither capacitive nor inductive
>coupling as those will induce noise at rising or falling edge only, not
>at the DC portion. It looks like there is some DC link between the
>digital and analogue but could not locate that from schematics or
>layout. Also, the circuit with no clock routed underneath works fine.
>
>Any thoughts ?
>
>Thanks
>
>Perry
>
>=======================================
>Perry Qu
>
>Design & Qualification, Alcatel Canada
>
>600 March Road, Ottawa ON, K2K 2E6
>
>DID: 613-7846720 Fax: 613-5993642
>
>Email: perry.qu@xxxxxxxxxxx
>
>=======================================
>
>
>
>
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