PSN-L Email List Message

Subject: Re: spike problem
From: John Popelish jpopelish@........
Date: Sun, 04 Jun 2006 13:02:56 -0400

J=F3n Fr=EDmann wrote:
> Hi
> I have connected the shield in the cable to the ground. Trugh a heating=

> oven (or how it is spelled / word over it) in the room, it uses hot
> water so it is ideal for this type of connection to the ground. That is=

> going to elimiate the shield problem chargeing up, at least it I hope i=
> does. The connection to the ground is farly good. There is paint on the=

> heating oven, but I don't think that is a problem.
> Now I only have to fix the larger spikes. Or replace the capacitor that=

> is possible leaking, this spikes happens on all the channels, so I am
> problay going to have to replace them all. If that can be done.
> Regards.=20

The important aspect of the cable ground is not that it actually be a=20
good Earth connection, but that it be the potential that the high gain=20
amplifier considers to be the zero volt reference.  If you tie the=20
cable shield to some actual Earth connection, you also need to tie the=20
amplifier zero volt reference to that same potential.

This difficulty in doing this may be that it is already grounded at=20
some other point (say, through the computer power plug), so that=20
adding this second ground contact will produce what is called a ground=20
loop.  The dual ground connection will try to equalize the=20
instantaneous potential at the two ground points by passing large=20
currents along the conductor that connects them.  This current will=20
inject (by transformer coupling) noise into the signal lines that=20
parallel the line that connects the two grounds.

This is why low noise systems are usually made with a very=20
specifically single point grounded system.


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