[SI-LIST] Re: digital to analogue noise coupling

Perry,

The waveshape is preserved if you have a resistive divider, or a
capacitive divider or an inductive divider.  From your stackup
I assume you have a capacitive divider case, where the coupling
capacitance creates a relatively wide-band voltage divider with
the input capacitance of the analog circuit.

Regards,
Istvan


Perry Qu wrote:

>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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