RE: Differential voltage probes 1 -> 3

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve.conner-at-optosci-dot-com>

I'm not talking about the match of the resistors inside the HV probe, I know
that can be good. I'm talking about the match of the source impedances
you're probing. For instance if your 'cold' probe is on a circuit ground
rail, that's coupled to AC ground, and your 'hot' probe is on a
high-impedance signal, like the cap of a 555 timer or something. The source
impedance of the signal adds to the resistance of the hot side probe divider
chain and screws up the CMRR.

I only know about this because it's a well known problem with balanced line
I/O in audio engineering. Anyway I just bought a Tek 7603 scope and I found
the 25MHz diff amp/comparator plugin on EBay for $25 :) I'm going to try
that out with a pair of matched-ish 100x probes, which will give a system
similar to what you proposed, and will report back to the list.

Steve C.



Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi,

 > > Anyway, I'm going off this whole diff amp thing, since I realized that
the
 > > CMRR depends on the matching of the two impedances (referred to the diff
 >amp
 > > box ground) you're probing, and they're unlikely to ever be matched in
 >real
 > > life. I'm going to investigate the optocoupler route instead.
 > >
 > > Steve C.

Matching the resistors may not be hard at all since other values could be
paralleled with the 1K's to "tweak" the resistance.  I'll have to test and
see, but I think it would be pretty easy to get a 0.1%+ match this way.