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

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From: Adrian Shiner (adrian.shiner@virgin.net)
Date: Sun Mar 26 2000 - 11:45:47 PST


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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