RE: differential probes

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hello,

I forgot there was stuff I was going to reply to  ;-)

comments below...
--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi Jimmy,
 >
 > I worked more on compensation tonight.  I am getting
 > real close.  Here is
 > the Tek probe (blue) and our probe (yellow):
 >
 > http://hot-streamer-dot-com/temp/03072701.gif
 >
 > There is a little bump in the beginning and a
 > "birdie" at 33MHz...  But
 > darn close for a pure resistive input probe ;-))  I
 > have 2.2K resistors in
 > the feedback so my gain is 10% high.  I'll try and
 > find a trim cap to
 > provide a proper compensation adjustment.  Don't
 > want to get too carried
 > away till I get the "real" resistors.
 >
 > I grounded Vocm but the darn offset is still very
 > sensitive to supply
 > voltage.  Probably have to bite it there and use
 > +-5V voltage
 > regulators...  A 1Meg trim pot and 100K resistor
 > going to Vocm provide a
 > very nice offset adjustment.
 >
 > Here is the exciting test setup:
 >
 > http://hot-streamer-dot-com/temp/P7270012.jpg
 >
 > Here is what the board looks like these days ;-))
 >
 > http://hot-streamer-dot-com/temp/P7270014.jpg
 >
 > More below...
 >
 > At 07:05 PM 7/27/2003 -0700, you wrote:
 > >Hi Terry,
 > >
 > >I am using the neutral for the MOV ground right
 > now,
 > >is there anything wrong with that?
 >
 > I the MOVs ever were to fire, they may have to dump
 > a lot of current into
 > the ground with a lot of voltage behind it.  I was
 > going to dump that back
 > into the coil's ground system rather than the
 > scope's ground.  Thus the
 > separate grounding...

Do you mean the RF ground? How much voltage would be
in the "ground"? I thought the scope would be able to
tolerate it in the neutral, but I guess if it's a
pulse cap, the ground would be a good portion of the
impedance.

 >
 > >more questions/comments below
 > >
 > >--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > > > Original poster: "Terry Fritz"
 > <teslalist-at-qwest-dot-net>
 > > >
 > > > Hi Jimmy,
 > > >
 > > > I got the boards today and was working on them
 > > > tonight.  I had to chop
 > > > things up a "little" :o)) but I think they will
 > work
 > > > very well.  Here are
 > > > the changes the boards could use so far.  I had
 > > > better write them down
 > > > before I forget ;-)
 > > >
 > > > 1.  Lets take the MOVs off the board.  It is
 > best if
 > > > the board just has
 > > > solder eyes for the HV wires.  We can just
 > solder
 > > > the MOVs to the fuse
 > > > holders and ground terminals rather than clog
 > the
 > > > board with them.
 > > >
 > > > 2.  The 1K feed back resistors need to be 2K.
 > >
 > >I noticed today that it is a 2000x probe. If I want
 > to
 > >make my other channel a 10x probe, then I would
 > change
 > >it to 200k right?
 >
 > I think 20K would be 10X and 200K would be 100X.
 > Right now it is 2000K of
 > combined input resistors with 2K of feed back
 > resistors for 1000X.  Be
 > careful of power dissipation in the input resistors
 > and all that.


I meant I wanted to change the feedback resistor to
200k. Would the high impedence be a problem?

 > > >
 > > > 3.  There is some offset and I think we can use
 > the
 > > > Vocm pin to fix that.
 > > >
 > >
 > >Grounding that pin worked perfectly for me.
 >
 > I still found that it was super sensitive to supply
 > voltage.  Regulated
 > supplies and an adjustment seem to be needed.
 >

I looked at it again, and found there to be some bias,
although much less than before. I don't know why I
didn't see it before.

 > > > 4.  We need the expensive 10nF caps after all.
 > > >
 > > > 5.  The 2K resistors need ~~50pF of
 > compensation.
 > > >
 > > > 6.  To go to a BNC cable, we need to put 50 ohm
 > in
 > > > series with the output
 > > > to match the cable.
 > > >
 > > > 7.  The feedback path was very noisy,  I had to
 > chop
 > > > it all out.  Need to
 > > > get ride of the ground plane near the input and
 > > > reposition the 2K resistors
 > > > right on the input.
 > >
 > >Can you explain this one? Does it have something to
 > do
 > >with capacitive coupling to the ground plane?
 >
 > Ohhh!! Yeahhh!!!  We need to eliminate as much input
 > "trace" as
 > possible.  Move the components to almost a star
 > junction.  Possibly mount
 > them on their sides to really pack them in...
 > Capacitance to ground and
 > anything else of the op-amp input node is "bad".
 >
 >
 > > > 8.  I replaced the 1 Meg 1/2 resistors with low
 > > > voltage surface mount
 > > > ones.  The big resistors were very noisy.  Hard
 > to
 > > > wait for DigiKey to get
 > > > the right ones...
 > >
 > >What makes the resistors noisy?
 >
 > The discrete resistors have a spiral element
 > (inductor) and they are "big"
 > so stray capacitance goofs with them.  They have all
 > kinds of
 > resonances.  Looks like a mini secondary coil at
 > high frequency.  Surface
 > mount types eliminate all that.
 >
 > Cheers,
 >
 >          Terry
 >
 >
 >
 >


=====
Jimmy