[TowerTalk] Low Inductance Ground Idea

[Jim Lux]

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.


>73s Tim EI8IC
>
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