On Fri, Aug 4, 2017 at 4:16 AM, Luke Kenneth Casson Leighton lkcl@lkcl.net wrote:
On Fri, Aug 4, 2017 at 8:51 AM, mike.valk@gmail.com mike.valk@gmail.com wrote:
2017-08-04 9:19 GMT+02:00 Luke Kenneth Casson Leighton lkcl@lkcl.net:
On Thu, Aug 3, 2017 at 11:53 PM, Richard Wilbur richard.wilbur@gmail.com wrote:
- Instead of using two separate anti-pads on signal vias, combine
them into an oval shared antipad (on every layer) to reduce parasitic capacitance.
oo. never heard of this practice. never heard of "anti-pads" either! so sorry, if you put some references i missed them.
[...]
Basically a barrier between a via and a passing layer. Something default to prevent shorting.
oh, ok - just a hole where you'd expect one to be :) didn't know its name was "anti-pad".
Sorry for not explaining very well and no references. Thank you Mike for adding both to the discussion.
With differentals have them combined so there is nothing in between them in a horizontal line/z axis as well.
turns out that the anti-pads from PADS are big enough to create a figure-8.
That's cool. The reason for an oval (or ellipse) shape instead of a figure-8, if you can muster an oval or ellipse, is that the sharp points greatly concentrate electric fields--leading to capacitance--which is pretty much the opposite effect of a smooth curve.
I'm glad to hear that successful HDMI layouts have been achieved using 5mil width, 5mil spacing for differential pairs. The reason I recommended 5.1mil width, 5mil spacing is because TI's geometry recommendation includes the condition width > spacing. Sounds like that is not a necessary condition for a working board.