[sdiy] Optoisolated USB ?

[John Luciani]

On 5/5/06, Seb Francis <seb at burnit.co.uk> wrote:

> It seems like I have 2 options:
>
> (1) Disconnect the scope's ground from mains ground
> Is this a bad idea?

A bad idea!!!

When you disconnect the scope ground from the mains ground
the chasis will float to a voltage that is dependent on the leakage
current of the
Y capacitors in the input power filter. Different scopes float to
different voltage
levels depending on the capacitors.

What you need is a differential amplifier for the scope. You could
also get an isolation
amplifier --- either an instrument or possibly build one. Check out
the Tektronix app notes.
If you have two channels on the scope you could try not connecting the
probe gnds and doing an A-B configuration. Not clean but may be
useful.

(* jcl *)

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