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

Larry, Istvan:

Thanks very much for your detailed explanation. You guys nailed the point of
huge RC in this scenario-- which is mentioned in Steve's earlier post but
does not strike me on the head. :) I can now reproduce the crosstalk in a
simple spice simulation, either through Istvan's "capacitance divider" or a
transmission line approach-- given the rise time and length of traces, I
think a lumped circuit is a good approximation. It's amazing how a tiny
little capacitive coupling can screw up the analogue circuit badly (the
mutual capacitance between trace and pads should be less than 1pF).

I can now understand why when we put a probe close to the clock trace on PCB
without touching it, we picked the same replica of the clock - a simple
capacitance divider effect since the probe has some capacitance and also
high impedance. The fact that the replica clock swing from -20mV to +30mV
also implied the RC discharging effect.

Also thank Steve and all others who helped me to understand this through
post here or private email. 

Regards

Perry


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