Re: [SI-LIST] : Analog ground connection on PCBoards

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From: Dave Graves (dave@ultracad.com)
Date: Mon Mar 27 2000 - 10:22:26 PST


Hi Yahuda,

>From a board layout perspective a there are couple of points in addition to
below;

--DO keep the supplies completely separate but the caution here is to
not
overlap your analog/digital planes,,, have your analog pwr mirror the
gnd
island otherwise risking coupling between dig/analog by way of the
planes.

--assure there is no trace routing traversing the plane splits. Contain
the traces within thier respective supply/gnd planes. If signals must
enter one plane area to the other do this at the point where the the
planes
themselves are joined.

-dave

At 08:45 PM 3/26/00 +0100, you wrote:
>Analog circuits are best segregated from digital circuits in the board
>layout. Analog circuits have many spurious modes of operation outside their
>nominal bandwidth of operation. A simple example is spurious RF
>rectification producing all sorts of difficult to cure problems. In a
>combined digital/analog board, there are many sources of RF coming from fast
>current changes. Long traces will act as antennae and high impedance
>circuits do not antenna impedance matching! I have an antenna amplifier in
>my study which has an 450mm long vertical antenna and a 150mm square
>capacitive top hat on it. It picks up signals from all over the world very
>effectively. Useable bandwidth is 100kHz to 200MHz.
>
>Grounding. Single point grounding is best every time to avoid ground loops
>and loop antenna effects. Use one ground point for the analog and one for
>the digital. That way you keep the noise of the digital power supply and
>circuits to a minimum in the analog. The common ground shoud be linked at
>the analog ground point.
>
>Power supplies..keep the analog and digital completely separate. It may cost
>a bit more to provide (e.g 2 off 5 volt supplies) but you will probably
>spend less time in the lab solving the common mode noise problems (& more
>time doing something more productive like time with your friends and
>family!).
>
>Hope this helps
>
>Adrian
>
>----- Original Message -----
>From: "Yehuda D. Yizraeli" <yehuda@zoran.co.il>
>To: "silist" <si-list@silab.eng.sun.com>
>Sent: 26 March 2000 16:40
>Subject: [SI-LIST] : Analog ground connection on PCBoards
>
>
>> Hi SI/Board experts,
>>
>>
>>
>>
>> We are designing a load board for a chip which contain a DAC and PLL on a
>> SOC. We were told that "...... its best for your analog to connect all
>> ground to one plane rather of having long GND traces on the board since
>the
>> long traces will make these signals noisy due to the aerial effect....."
>>
>> Since its opposite to the theories I know of, and the data is from board
>> designers (might be they are not well experienced in shielding these
>> signals), I would like to hear others' opinion.
>>
>>
>> Any experienced based answer with/without pointers to articles will be
>> blessed with THANKS!!!
>>
>>
>>
>>
>>
>>
>> regards, yehuda
>>
>>
>> -------------------------------------------------------------------
>> Yehuda D. Yizraeli
>>
>> Zoran Microelectronics Ltd. E-mail : yehuda@zoran.co.il
>> Advanced Technology Center Direct Tel: 972-4-85-45-795
>> P.O.B. 2495, Haifa 31204, Operator : 972-4-85-45-777
>> ISRAEL Cellular : 972-52-556-335
>> http://www.zoran.com Fax : 972-4-8-551-550
>> -------------------------------------------------------------------
>>
>>
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>>
>> Problems are only opportunities in work clothes.
>> -- Henry J. Kaiser
>>
>>
>>
>>
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>>
>>
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>

Dave Graves
UltraCAD Design
http://www.UltraCAD.com

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