[SI-LIST] Re: Power planes, ground cutout and guard traces
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: han.guobing@xxxxxxxxx, dd@xxxxxxxxxxx
- Date: Wed, 12 Nov 2014 08:19:33 -0500
Please note that grounding a shield trace at both ends vs only one end
shifts the
resonance frequencies of the guard trace from quarter-wave to half-wave,
which
is a factor of two increase. With today's rise times it is usually
still not enough.
Empirical data shows that to avoid excess ringing, the round trip delay
between
adjacent grounding vias should not be more than the rise time of the
main signal(s).
Assuming 100ps rise time, adjacent grounding vias should be closer than
one third
of an inch or less than one centimeter.
Regards,
Istvan Novak
Oracle
On 11/12/2014 2:19 AM, Han, Guobing wrote:
> Hi Michele and Danny,
> Just one comment.
> Be careful about the guarding traces, which may cause extra resonance,
> especially when one end is open/floating.
> If you use guarding traces, it's recommend to ground them at both ends and
> better add one GND via every wavelength/10.
>
> Thanks,
> Robin
>
> 2014-11-05 18:00 GMT+08:00 Danny Damhave <dd@xxxxxxxxxxx>:
>
>> Hi Michele,
>> 1) There is no "0 Ohm resistors". Some are closer to "0" than others but
>> expect a voltage drop.
>> Consider to design a special footprint with a short track between the
>> pads, keep the component not mounted
>> and cut this track for debugging and mount one of the better "0 Ohm
>> resistors" when needed.
>> 2) Be very carefull with cutouts, they increase crosstalk to nearby traces
>> a lot because of the large loop they introduce.
>> 3) Bogatin and Simonovich has written an excellent paper "Dramatic Noise
>> Reduction using Guard Traces with Optimized Shorting Vias".
>> But be aware that this paper and most other advices about guard traces are
>> based on a 50ohm (or close) environment. Maybe your sensitive analog traces
>> are high impedance
>> traces and then the conclusion is different and you will probably get more
>> from guard traces.
>> Best regards
>> Danny Damhave
>> Damhave Systems ApS
>>
>>
>>
>> On 05/11/2014, at 10.24, Michele Manotti <michele.manotti@xxxxxxxxx>
>> wrote:
>>
>>> Hi experts,
>>> I'm designing a mixed signal board where the analog part is very critical
>>> so noise and disturbances has to be minimized. I have three questions for
>>> you:
>>>
>>> 1) All the board is powered with LDOs and between regulators' output and
>>> power planes I'm about to place 0 Ohm resistor to be able to disconnect
>> one
>>> rail at a time in case of unwanted short due to production error. Does
>> this
>>> have counter effects? I mean having a wide power plane under loads and
>>> decoupling capacitors connected with a narrow path (the resistor) to
>>> regulators output.
>>>
>>> 2) Analog and digital parts share the same ground. In the common ground
>>> plane I would insert a cut out between the two parts and make the two
>>> grounds (actually the same) connection under the ADC. Just to be clear:
>>> analog part is in upper right corner, digital one in lower right corner
>> and
>>> ADC in the left side and so the ground plane make a sort of C. Doing this
>>> way some digital traces (switching eventually at tens of MHz) in inner
>>> layers would cross the ground cutout (I can't move or remove them).
>> Could
>>> this approach add more noise instead of reducing it? Would you suggest
>>> better techniques?
>>>
>>> 3) On top and bottom layers I have space to insert guard traces between
>>> analog and digital parts in order to reduce crosstalk. What do you think
>> is
>>> the best way to accomplish this (leave trace floating, grounding it, how
>>> many vias)?
>>>
>>> Thanks a lot,
>>>
>>> Michele
>>>
>>>
>>>
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