[TowerTalk] Low Inductance Ground Idea

[Tim Makins, EI8IC]

Thanks for the explanation, Jim.

73s Tim EI8IC



----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: "Tim Makins, EI8IC" <contesting at eircom.net>; "TowerTalk"
<towertalk at contesting.com>
Sent: Thursday, May 13, 2004 10:20 PM
Subject: Re: [TowerTalk] Low Inductance Ground Idea


> At 06:42 PM 5/13/2004 +0100, Tim Makins, EI8IC wrote:
>
> >----- Original Message -----
> >From: "Jim Lux" <jimlux at earthlink.net>
> >Subject: RE: [TowerTalk] Low Inductance Ground Idea
> > >
> > > I've certainly had the unpleasant experience of having racks of
equipment
> > > where the grounding was bad, so one rack floated up to 60-70 VAC
relative
> > > to a "good ground" just from capacitive coupling between AC line and
the
> > > equipment.  This is not a good thing when you have low impedance
> >electrodes
> > > attached to your head and you touch the rack.
> >
> >
> >OK, so you've got me all curious !! Why did you have low impedance
> >electrodes
> >attached to your head ???
>
>
> Back in the 70s, I was doing research on brain waves, so we had high gain
> differential amplifiers hooked up, and a ground/guard electrode to reduce
> the common mode signal that the diff amp would have to reject.  Typical
> electrode impedances (at 100 Hz) would be around 1K ohm, so even if the
> line was leaking through 100K, you'd feel it (threshold of sensitivity is
> around 1 mA).  In this case, I think the leakage path was through some 0.1
> uF caps hooked from line to ground(and case) for "transient suppression"
> inside the equipment.  The ground pin in the cord wasn't connected (or had
> failed), so the equipment case was hot, relative to the actual ground.
>
> In the last 20-30 years, medical equipment designers have become very
aware
> of such paths, and patient isolation is much better. (and they have clear
> plugs so you can visually inspect the connections, too!).
>
> In any case, measuring microvolt signals from 1-300 Hz, and rejecting 60Hz
> interference is quite the challenge.  Ground loops have real significance,
> when you're trying to get 100 dB+ CMRR.
>
>
> >> >
> >_______________________________________________
> >
> >See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
> >Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
> >any questions and ask for Sherman, W2FLA.
> >
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> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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