Date: Sun, 03 Mar 2002 11:20:40 -0500

Author: Geoff Nunes

Subject: Re: F.G. with floating voltage


David Kardelis wrote:
> I have been using standard FG but put it on a adaptor that is used for
> attaching 3 prong connectors to older 2 prong outlets. This leave the FG
> floating. Not elegant but it works.

Please don't do this. Not only inelegant, but rather unsafe.

Presumably Yalan's problem is that an oscilloscope or something
introduces a second ground somewhere in the circuit? If that is the
case, then this fix, or using an isolation transformer (which amounts to
the same thing and is offering no safety when used like this) is sort of
but only when the demonstrator is the Prof. who has been (we hope!)
fully informed of the situation.

Do it for a lab set up and you're just asking for trouble.

Leave the signal generator grounded and use a differential amplifier on
the 'scope input to prevent it from introducing a second ground. You
can put an Analog Devices AD620 (or similar) into a box and power it
with a couple of batteries for just a few dollars a pop.

If the output of the signal generator MUST float, then put a
small-signal isolation tranformer on the _output_ of the signal
generator. The bad news is that these transformers belong to the same
era as those floating output signal generators, and are hard to find.
You can get them from Jensen Audio, since they are still used in
high-end audio recording equipment, but they are painfully expensive. I
have not found a cheaper source.

-Geoff Nunes