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Differential probe construction

Discussion in 'Electronic Equipment' started by VMI, Apr 17, 2004.

  1. VMI

    VMI Guest

    Hi,
    I looked around but nowhere I whas able to find a schematic of a
    differantial probe.
    It's main use is to measure with a scope on smpsu's and the like.
    I know we can remove the ground lead from the scope, but where then is the
    protection left in the case of...
    Anny ideas or info?
    Thanks,
    Marco
     
  2. Bill Janssen

    Bill Janssen Guest

    If you have one of the newer two channel scopes that feature is built in.

    Bill K7NOM
     
  3. mike

    mike Guest

    Biggest problem is getting enough common mode rejection and sensitivity.
    A dual trace scope usually has a subtraction feature, but you have to
    keep the whole input inside the dynamic range of each channel. Then
    when you subtract them, you get the thing you really want to look at,
    but it's really small. And even excellent CMRR at the scale factor
    you need to use ain't enough by the time you subtract them.

    If you try to crank up the gain, the individual
    channels saturate and make a mess of things.

    Even if the scope has super CMRR, you still need perfect balance
    in the probing system. Making a homebrew differential probe will
    be an exercise in futility.

    What you really need is a probe that's isolated so you can hook
    the "ground lead" of a single channel on something that's not really
    ground. But wait, such a thing exists.
    And I even have one for sale here:

    There's a Tektronix A6902A for sale at:
    http://nm7u.tripod.com/homepage/te.html

    mike


    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    Yaesu FTV901R Transverter, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  4. VMI

    VMI Guest

    Hi Mike,
    Thanks for the info.
    It's still not what I'me looking for:
    Need to be used on SMPSU's and Power amps in bridged mode.
    The use of 2 channels is not such a bad idea, but what if you forget that
    you may not connect the ground?
    It happens faster than one should think (I know...)
    So, a probe that can easely connected to such a load and that do not cost to
    much.
    A schematic is preffered.
    Thanks,
    Marco
     
  5. mike

    mike Guest

    You have the same problem when you drive your car. If you forget
    to keep it on the road, you will have a crash. Remove the ground leads
    and you won't have anything to connect to ground.

    You can also buy differential input plugins that use two probes.
    They're much better than two separate amplifiers, but still have
    resolution problems if the dc level is much greater than the amplitude
    of the signal.

    mike


    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    Yaesu FTV901R Transverter, 30pS pulser
    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
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