[SI-LIST] Re: digital to analogue noise coupling
- From: steve weir <weirsi@xxxxxxxxxx>
- To: perry.qu@xxxxxxxxxxx, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 20 Jun 2005 06:02:52 -0700
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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