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From: John Woodgate
Subject: Re: Bypass RS-485 shield--ESD problem??
Date: Wed, 2 Oct 2002 21:33:33 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Wed, 2 Oct 2002 21:47:45 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Jim H wrote (in
) about 'Bypass RS-485
shield--ESD problem??', on Tue, 1 Oct 2002:
>I am getting RF leakage into my controller (Maxim 1480B based) from
>the shield of the signal cable on my 8 node 1200 Meter Rs 485 network.
> Shield is grounded thru 100 Ohms per datasheet. I will add a ferrite
>bead at the control end to try to keep the unwanted RF (80-130
Is this 80 -130 MHz coming from the control equipment or from external
sources? A series bead may not do much; it's in series with the
inductance of the shield, which at 100 MHz is very significant. And of
course you network is many wavelengths long, so there are transmission-
Is your shield at the *control* end connected to the *outside* of the
enclosure? If it connected inside, it's an antenna and downlead, just
right for liberating 100 MHz internal stuff that the enclosure is there
to confine. It's the same at the other nodes; shield to *outside* of
> If that doesn't work - I would like to bypass the
>shield at the other nodes with a 100 nf cap - but won't this become a
>part of the ESD picture? I need to preserve my 15 KV ESD rating for
I don't at present understand your reference to ESD. What precisely are
you concerned about?
100nF is FAR too big for decoupling 100 MHz. Even a 1nF cap needs VERY
short leads so as not to be resonant in that frequency range. Multilayer
discoidal capacitors (AVX, for example) behave differently if installed
correctly. Even a 10 nF has a low impedance up to 1 GHz (I've measured).
They don't behave as capacitors at very high frequencies; more like
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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