[SI-LIST] digital to analogue noise coupling

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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