From: Winfield Hill
Subject: Re: ground loops, optoisolators, and ADC noise, oh my!
Date: 21 Jan 2003 11:52:13 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> I've been looking at optoisolators but they're expensive and slow.
> Plus, they all show propagation times that don't seem that suitable --
> they vary markedly with temperature and input current, all of which
> would contribute (50-100 nS in some cases) of jitter to the CONVST.
Opto-isolators aren't all THAT slow, for example, Agilent's HCPL-7720
has a typical delay of about 20ns, and jitter is a fraction of that.
You can also use optically-coupled fibre components, such as Agilent's
HFBR-14xx and HFBR-28xx series, some of which work to 175 MBd, with
sub-ns PWD "jitter."
Even though parts like that may not be suitable for you, other high-
speed optical components other can suggest may well be.
> In one of the analog devices app notes it is mentioned that people have
> used differential signalling (Such as ECL) for transmitting this sort
> of pulse... I'm curious as to how differential signalling gets around
> the problem of current return, and if anyone has used this or any
> other technique in the past to provide sample clock isolation...
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.