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From: "Bill Sloman"
Subject: Re: ground loops, optoisolators, and ADC noise, oh my!
Date: Wed, 22 Jan 2003 01:07:03 +0100
Organization: Planet Internet
NNTP-Posting-Date: 22 Jan 2003 00:07:20 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Winfield Hill" wrote in message
> firstname.lastname@example.org wrote...
> Part of the idea is to send fully balanced signals so return currents
> are dramatically minimized. Of course, shielded transformers can also
> be used with LVDS to completely eliminate return currents.
One of the nice things about LVDS and ECL signallingis that the voltage
swing is much lower than TTL - classic biploar TTL swings from about 0.4V to
2.4V, while HCT swings the whole 5V if it gets the time.
ECL is only 800mV and - IIRR - LVDS is in the same ball-park. Using fully
balanced ECL/LVDS signals cancels out a large proportion of a rather smaller
For this sort of application , conventional shielded transformers are rather
slow. I'm sure that what Win had in mind are transmission line transformers,
which are flat up to the frequency where the windings are a quarter
wavelength long - an insight that I owe to Win.
You can wind something that will transfer 150MHz components using a twisted
pair of 38 gauge enamelled copper wire as your transmission line. We used
quite a few turns on an ungapped RM6 core, and the low frequency limit was
about 50kHz. Many fewer turns of 1mm OD minature coax on the same core was
flat up to 500MHz.
The interwinding capacitance was about 100pF.
Bill Sloman, Nijmegen
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