[SI-LIST] Re: GND Separation for Analog & Digital circuits
- From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- To: geva@xxxxxxxxxxx
- Date: Fri, 20 Feb 2004 22:18:40 -0800
All,
For these types of problems, visualize where the fields will go and use
that to determine how to isolate them. At audio and video frequencies,
remember that fields do penetrate planes, and that significant noise can
occur when digital signals pass underneath analog sections separated by
a plane. For good isolation, be sure that no digital signal flows
through the analog section on any nearby layer, even if it is separated
by a plane. If this is unavoidable, use 1 oz or 2 oz copper instead of
1/2 oz copper, to limit field penetration through the metal. Make sure
that power planes and traces are totally isolated from noise sources,
and digital power sources, with large amounts of separation (> 20 X the
dielectric thickness between power and ground planes.) Use 2-D field
solvers to determine the crosstalk levels (noise) that will occur.
These problems are not hard and can be easily be approached with general
purpose quasi-static field solvers that can solve for field penetration
thru metals, like Ansoft 2D.
Visualize the fields.
Be the fields.
And all will be well.
scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Geva wrote:
>Hello Maheshwari,
>
>To answer your question on separate analog and digital GND for Video and
>digital signals;
>I had very similar problem of CATV receiver with video at 43 and 5 MHz,
>combined with digital signals on same board.
>In the PCB layout design we completely separated the 2 grounds and later
>on, in the lab, we found out the optimum
>locations of connections between the digital GND and the Analog GND, and how
>many connections needed, for the best performance.
>
>in order to get the best signal to noise ratio, we used more than one
>connections between grounds.
>
>You may come with many theoretical conclusions about one or more connection,
>but the test will specified the best answer for you.
>
>Ehood Geva
>
>
>----- Original Message -----
>From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
>To: "Zhangkun" <zhang_kun@xxxxxxxxxx>; <maheshwari@xxxxxxxxxx>;
><si-list@xxxxxxxxxxxxx>
>Sent: Friday, February 20, 2004 9:43 AM
>Subject: [SI-LIST] Re: Separate GND for Analog & Digital
>
>
>
>
>>Zhangkun,
>>
>>Thanks for sharing your results. It supports the concept of keeping the
>>noise out of the analog section by careful routing of those traces.
>>
>>Lee
>>
>>
>>
>>
>>>[Original Message]
>>>From: Zhangkun <zhang_kun@xxxxxxxxxx>
>>>To: <maheshwari@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
>>>Date: 2/20/2004 9:13:05 AM
>>>Subject: [SI-LIST] Re: Separate GND for Analog & Digital
>>>
>>>Dear Maheshwari
>>>
>>>We have come cross the simular problem. The critial point is to prevent
>>>
>>>
>>return path of digital signal disturbing the analog signal.
>>
>>
>>>In one of our PCB, there is too much digital noise in analog signal. We
>>>
>>>
>>have to repair it. How to seperate the ground seemed to be the most
>>important problem. One is to seperate the ground and to connect the
>>
>>
>digital
>
>
>>ground and analog ground at one point. The other is not to seperate the
>>ground. In order to make sure that the product is OK, we made two boards,
>>seperated and not seperated. In the test, both boards are OK.
>>
>>
>>>It has nothing to do with seperating ground or not. Being care of the
>>>
>>>
>>digital return path is the most important. Some CAD tools could do such
>>simulation.
>>
>>
>>>I hope this will help.
>>>
>>>Best Regards
>>>
>>>Zhangkun
>>>2004.2.20
>>>
>>>----- Original Message -----
>>>From: "Maheshwari.P" <maheshwari@xxxxxxxxxx>
>>>To: <si-list@xxxxxxxxxxxxx>
>>>Sent: Thursday, February 19, 2004 7:41 PM
>>>Subject: [SI-LIST] Separate GND for Analog & Digital
>>>
>>>
>>>
>>>
>>>>Hi All,
>>>>
>>>>I am designing boards which contain Audio and Video codecs. I need to
>>>>
>>>>
>>pass
>>
>>
>>>>analog signal to the corresponding decoders without much noise.
>>>>
>>>>Is it better to have separate analog and digital GND?
>>>>or
>>>>
>>>>Is it better to have a solid GND and have separate analog and digital
>>>>sections?
>>>>
>>>>Which of the either way will give better results in terms of audio and
>>>>
>>>>
>>video
>>
>>
>>>>quality?
>>>>
>>>>Thanks in advance,
>>>>Maheshwari
>>>>
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>
>
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