Hi Forrest,

A couple of years ago at work I bought two $500 25MHz differential
scope probes from www.probemaster.com. They were one of the best
purchases I ever made. I often use them now even when ordinary scope
probes would be acceptable because the signals are so much cleaner
(not just because of the narrower bandwidth - I mean that the signals
are a more faithful representation of the actual signal).

These type of probes are NOT cheap - they are very specialized, rugged
differential amplifiers. You probably cannot just buy a diff amp IC
with these specs. I suspect that, inside the probe, there are some
discrete transistor input stages with matched transistors and
resistors, followed by some high-speed op-amps, all connected in an
instrumentation amplifier topology.

For home use, I did manage to get an old (1970s vintage) 100MHz Tek
diff probe for about $100. The downside is that it is bulky (has a
separate power supply, probe, and amplifier box). The going rate for
used modern diff probes seems to be about $300. New they are in the
$500 range - this is for 20 to 30MHz BW. The really high end ones (GHz
BW) are thousands of $.

Sean


On Thu, Aug 19, 2010 at 12:00 AM, Forrest W Christian
<forrestc@imach.com> wrote:
> =A0My most recent project has created a new challenge for me. =A0 I use a
> TDS2014 4 Channel DSO on the bench, and to date, just some good garden
> variety 1X/10X scope probes (basically decent quality but not really
> name-brand probes - maybe pomona or tenma or something like that).
>
> This new project involves putting 120us holes in 60VDC. =A0The stuff is
> basically done at this point, but I think I've spent almost more time
> dealing with scope grounding issues than actually solving problems.
> Switching 60VDC off and on that quickly obviously creates some
> interesting inductive possibilities. =A0For instance, earlier today I was
> seeing some stray spikes on one of the signals being monitored. =A0 Pulle=
d
> the scope probe out of it's 'grabber clip thingy (yes technical I
> know)', and touched the tip to the grounding clip (you know, the one
> with about 3 inches of wire attached directly to the probe itself).
> Amazingly, the spikes were *still* there. =A0 Ended up disconnecting all
> the other grounds on the other probes and referencing only to that one
> to verify that signal was clean. =A0Of course, then all the other signals
> weren't clean anymore - at least not on the scope.
>
> So, I figured to fix this in the future, I'd spend a bit and get some
> differential scope probes. =A0 So I glanced on ebay and google for costs
> and availability - and in the process about fell out of my chair in
> shock. =A0 Maybe there's a decent one out there at a reasonable cost that
> I'll find, but looks like they're few and far between.
>
> So, before I either just suffer - or go out and buy a few isolation
> amplifier ICs and build my own, I figured I'd ask on here if there are
> either some tricks I'm missing (I.E. maybe it's the scope probes I'm
> using - a $30-50 probe just isn't good enough, OR there is some other
> way to get a good picture of what a signal looks like referenced to
> ground near the chip producing it - when you have 4 possible grounding
> points.
>
> Ideas?
>
> -forrest
>
>
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