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Re: ScopingSSTC



Original poster: Esondrmn@xxxxxxx
Terry,

Don't today's scopes have a differential mode select? I haven't used a scope in a long time - but when I needed to scope signals that had a 60 hz reference I just used a probe from each channel and set it to differential mode. Of course you need a two channel scope to do this.

Ed Sonderman

In a message dated 12/14/2005 2:41:04 PM Pacific Standard Time, tesla@xxxxxxxxxx writes:
Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

I have run a lot of scopes with the ground hot as you mention (the
scope is grounded though the AC line Ground so you need to defeat
that).  The scope chassis and all adds a lot of capacitive loading to
the circuit which may very well be bad for gate drive
measurements.  Also, the scope's power supplies are still referenced
to ground through the AC lines.  The isolation at that point is not
very well guaranteed.  So scope loading and isolation from the AC
line are issues.  Tektronix scopes seem to be the "luckiest"...  Dig
through the specs sheets where they give clues about how hot you can run.

Also, try not to run the scope chassis on the RF side but rather the
fixed voltage rail side so the chassis is not oscillating at
250kHz.  Scopes don't like that...

It is a pretty messy business really.  I have also pried many scopes
off grounded metal workbenches when they slip a little off whatever
we had them on and they arc weld themselves to the bench.  It usually
does not hurt the scope "much" though!  We also had a bunch of "extra
scopes" and the repair costs were "acceptable" at the time.  I would
never run any scope "I" paid for hot...

Of course, don't touch the darn thing...  Some little hand held
scopes are battery run and fully isolated too and cost less than a
good dif probe!

http://www.amazon.com/gp/product/B0007ZPNH8/ref=nosim/104-6863387-9336701?n=228013

I suppose, someday, Amazon will sell CD caps too :o)))

We did toy with the idea of making our own probes at one time:

http://hot-streamer.com/temp/DiffProbe03.gif

But we got all tied up in trying to make them have super high
bandwidth and stuff and it never really went anywhere...  We should
have kept it simple and doable...

Cheers,

        Terry