RE: Differential voltage probes 2

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

Original poster: "Wall Richard Wayne by way of Terry Fritz <teslalist-at-qwest-dot-net>" <rwall-at-ix-dot-netcom-dot-com>

Terry,list,

Here is a quick diagram of a simple single IC four op amp differential
instrumentation op amp based on a quad fet op amp.  The inputs are buffered
and balanced.  Any input bias can be fairly well balanced with variable R
on non-inverting input
IC-3.  IC-4 reinverts output, allows amplification of small signals and
buffers output.  (actually IC-4 can be left out, but signal will be
inverted).  If you like TL082 (dual) then TL084 (quad) j-fet op amp with a
few extra resistors will do the trick here.  Try it both ways with a single
op amp and four op amps and you'll find a big difference.

http://hot-streamer-dot-com/temp/INST%20OP%20AMP%204.jpg

I agree -- keep it simple at first then get fancy later.  Maybe Dan could
design a pcb for us later.

RWW




 > I am thinking we need two probe designs.  One super simple one like I
 > suggested.  It would be great for many things like Steve's test this
 > morning (Steve says the scope just lost a few ranges on one channel, so
it
 > should only be minor resistor damage :-))  A very simple probe would
 > eliminate floating scopes and clipping the ground lead to the hot line
 > errors so common when scoping AC stuff...  Very easy for anyone to make.
 >
 > But maybe we also need a higher performance design too?  We could maybe
 > spend a few more bucks on a true higher bandwidth design that had the
CMRR,
 > offsets, opto-isolation, good impedances and all that fixed for more
 > serious work.  PCBs and parts kits re no problem for those that want
 > something "better".  Probably be surface mount...