Tony,
Thanks for the great ideas!
%>
%>Hi,
%>One way to do this would be to line the gap with decoupling caps
%>in the vicinity where the traces cross, on either side of the gap. The
%>values of the caps would be chosen so that they self-resonate
%>(due to self AND via series inductance) in the problem frequency range.
%>But 189 MHz is getting up there in frequency. Assuming 8 nH (3 nH
%>per via and 2 nH cap self inductance) a decoupling cap value 100pF is
%>in the ballpark.
I hadn't given any thought to this problem, but now that you mention it, I
have found a spot in Intel's P6 application notes where they recommend doing
the same thing. The problem of course is in trying to get them at the
resonant frequency and there is a chance that the next turn of the card (a
major change) will have a problem at a different harmonic. Still, it's worth
a try.
%>
%>I don't really like that approach. Instead, I wonder why the route is
%>allowing traces to jump the gap between analog and digital power.
%>I would think that most of the shared circuitry between analog and digital
%>power is within the DAC itself and that if the power planes are well placed
%>and of the proper shape, there isn't much external route on the board that
%>needs to intermingle between the two power areas, especially not a clock
%>signal.
We have designed the board so that our DAC power plane doesn't have this
problem. I learned that the hard way! However, the other island involved is
a 3.3V island in the 5V power plane.
%>
%>Even if power and ground are solid planes, I don't like that layers 1
%>and 8 do not have a return layer adjacent to them. Again, loop area is
%>greater than the optimum value unless you use significant return route
%>on the same layer adjacent to to signal route for any critical signal on
%>these
%>top and bottom layers. If more power and ground layers are not an option,
%>I would rather see the signal layer count DECREASE by two layers
%>using a 6 layer board with your stackup or DECREASE by four layers
%>with the power and ground layers in the middle to take advantage
%>of better distributed capacitance board decoupling.
I agree completely. I have sent a note out saying that the only good
solution is to get the number of signal layers down to four and to add two
additional ground planes.
%>
%>You will get better high frequency decoupling if power and ground
%>are adjacent in the stackup. This is due to the distributed capacitance
%>created by the adjacent planes due to the more closely spaced planes
%>(capacitor "plates"). With the number of signal layers you
%>are using, you would exacerbate the problems of adjacent layer
%>returns if you went to this type of structure. So considering the number
%>of power and ground planes vs. signal layers you have, your current
%>stackup with its deficiencies are your most logical option. Again, for this
%>many signal layers, I would like to see one more power and one more
%>ground layer with the following stackup.
%>
%> S
%> S
%> G
%> P
%> S
%> S
%> P
%> G
%> S
%> S
I wonder how this compares with this type of stackup?
S
G
S
S
P
G
S
S
G
S
%>
%>I would keep high speed route off of the top and bottom layers as
%>much as possible and would resize the analog and digital planes
%>to avoid high speed signal traces jumping the gap. I would also
%>use vias around the perimeter of the board to stitch the ground planes
%>together every 0.1" or so.
I never thought of lining the perimeter of the board with caps. Is that
how you do it?
%>
%>Furthermore, you can retain the original board cost and improve things
%>further if you can squeeze the signal lines into 4 layers as follows:
%>
%> S
%> G
%> P
%> S
%> S
%> P
%> G
%> S
I wonder how that compares with this kind of stack up?
S
G
S
P
G
S
G
S
I guess the answer depends partially on how well the power plane acts as a
return.
%>
%>Now having said all of that, the above concerns may not be your
%>problem at all and changing all of this may not help. But hey,
%>consider the advice worth what you paid for it!
%>
%>Regards,
%>[email protected]
%>
%> ----------
Thanks Again,
Max
