|To:||"Tim Makins, EI8IC" <email@example.com>,"TowerTalk" <firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] Low Inductance Ground Idea|
|From:||Jim Lux <email@example.com>|
|Date:||Thu, 13 May 2004 14:20:03 -0700|
At 06:42 PM 5/13/2004 +0100, Tim Makins, EI8IC wrote:|
----- Original Message ----- From: "Jim Lux" <firstname.lastname@example.org> 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.
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
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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