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Making a differential scope probe, how tough?

Discussion in 'Electronic Design' started by Chris Carlen, Jun 20, 2006.

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  1. Chris Carlen

    Chris Carlen Guest


    I've been wondering about the feasibility of making a differential scope
    probe that can work as well as or better than the low-end ones on the
    market. Since it would be a home-made thing, it wouldn't need to have
    any certs.

    I'd like to achieve these specs:

    25MHz bandwidth.

    Less than 2% error introduced in a 10V gate drive signal when swinging
    +/-100V at moderate switching speeds of up to 100kHz. So if the rise of
    the switching is 100ns or so, that's spectral content at about 3.5MHz.
    Let's say CMRR of

    20*log[200/(0.02*10)] = 60dB at 3.5MHz.

    Let's shoot for 65dB at 1MHz and 80dB at 100kHz and below.

    Input: 1 or more Megohm differential input R and =>1 Megohm common mode
    input R. Less than about 13pF input C.

    This CMRR spec is moderately better than the 50-60dB at 200kHz-1MHz
    typical of the stuff I'm finding on the market.

    No range switching, just a hardwired unit for each input division ratio.

    I'd like to have an input division of 1/20 for a 100V common mode and
    100V differential voltage readable range option (with tolerance of 200V
    max at either input without breaking), as well as possibly a unity gain
    input for low level stuff.

    A second stage of amplification could offer 1x, 10x, or 100x to make it
    possible to get up to about 10uV/div sensitivity on a typical DSO.

    The package would be something like a Pomona box with the differential
    input being perhaps just a two pin header sticking out of a hole at one
    end to which short test leads could be attached. To avoid noise and
    keep things simple, a pair of 9V batteries for power or perhaps a cabled
    source of +/- DC voltage.

    Can this be done without using a discrete differential input stage (ie.,
    regular op-amps) ?

    Is the bulk of the difficulty focussed on matching input parasitic
    capacitances to get the CMRR? Careful layout and a few trimmer
    components should make this doable, no?

    Thanks for input.

    Good day!

    Christopher R. Carlen
    Principal Laser&Electronics Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and
    "BOGUS" from email address to reply.
  2. John Larkin

    John Larkin Guest

    Why not battery power, fully floating, and a fiberoptic link to the
    receiver box on the scope? Hmmm, maybe I should manufacture those!

  3. NO!
    [Scratching the old memory cells]
    "Medical electronics" developing an input stage for ECG recorder. The
    signal is in range of 0.5-5 mV on a human body noisy with about 1V line
    frequency voltage. You need about 8 hands to trim all the trimmers each
    time you blink your eye. So the "expensive?" solution was to go into
    microelectonic solution of first amplifier stage with the input and
    feedback resistors monolitic with the division ratios certified by
    Vishay to x/ppm. Only then the rest of the circuit became trivial.
    So if you can get hold of such a stage it will save you a lot of
    But it is your "craziness", so good luck!

    "We always learn!" - a Hassidic proverb.

    Slack user from Ulladulla.
  4. Chris Carlen

    Chris Carlen Guest

    Your probably capable of just such a thing, which would be pretty cool.
    Of course it would cost a bundle.

    One question, does having an ill defined relationship to ground make the
    input behavior hard to tune?

    What proportion of the typical gadget's cost is composed of regulatory
    compliance? Ie., UL, FCC, EMC, CE, RoHS, IEC, etc. certification costs,
    and/or the costs in time or consultant fees to engineer compliance?

    Good day!

    Christopher R. Carlen
    Principal Laser&Electronics Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and
    "BOGUS" from email address to reply.
  5. That's the killer.

    If you were happy with about 100KHz, Analog Devices and Burr Brown (TI)
    do some nice isolation amps like this:,,759_783_AD215,00.html

    Dave :)
  6. John  Larkin

    John Larkin Guest

    Not really. 850 nm VCSEL lasers and silicon photodiodes are fairly
    The circuit would still have a local ground/Farady cage, probably a
    metal box or equivalent, tucked inside a plastic case.
    I wouldn't do any of that, and none are needed for the US market

  7. Chris Carlen

    Chris Carlen Guest

    Sometimes 100kHz is pretty useful. I've still rarely been upset by only
    having a 100kHz current probe at my disposal.

    It isn't sounding very encouraging about the 25MHz diff voltage probe

    Good day!

    Christopher R. Carlen
    Principal Laser&Electronics Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and
    "BOGUS" from email address to reply.
  8. Chris Carlen

    Chris Carlen Guest

    I wasn't talking about the parts cost. I suppose if you are correct
    about not needing to address compliance issues, then it might not be so bad.

    It is a very cool idea though. Now tell me, would it make more sense to
    digitize each input separately, then subtract digitally, or actually
    implement an analog differential amp, then digitize a single ended result?

    What rate of oversampling would you use? I'd like to see at least 10x
    if not 20x. At these speeds, it wouldn't be easy to get more than 8-10
    bits. Hmm, maybe that means the analog diff front end is still needed.

    Heck, even I can do an optical data link. Done one before with free
    space transmission off a running engine piston!

    But discrete analog and high-end stuff isn't my thing.

    Interesting. None are needed for the "instrument" market, or any
    market, such as consumer products? Are these certs all just voluntary?

    Wait a minute, I thought at least FCC part 15 or something was required
    for anything with a clock running at more than some dismal speed?

    Thanks for comments.

    Good day!

    Christopher R. Carlen
    Principal Laser&Electronics Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and
    "BOGUS" from email address to reply.
  9. Paul Mathews

    Paul Mathews Guest

    Are you wanting high voltage differential probe or logic level diff
    probe...2 very different animals. Assuming the former, it's not too
    difficult to do something like Tek 5200. You attenuate both inputs with
    matched resistors to get the voltages down to reasonable values,
    connect to an instrumentation amplifier and a line driver. Tek 5200
    uses readily available Intersil ICs, if I recall correctly.
    Paul Mathews
  10. If you're interested in measuring the source to gate voltage, you may
    be in luck.

    If the source is a relatively low-impedance point, you can just use a
    single-ended amplifier, that eliminates HALF of the amplifiers and
    balance adjustments.


    Another way is to just buy an old Tek 465B with battery pack, hang it
    from the ceiling with fishing-line, and just use it with a regular
    scope probe, hooking the ground lead to the source. Should be less
    than 10pf to ground.


  11. John  Larkin

    John Larkin Guest

    If it's fully floating, there's no "differential" input; there's the
    input, and there's the return, the "ground" clip, the circuit common
    of the probe. And I wouldn't digitize... probably go FM, which is how
    the Tek scopes do it, I think.

    A differential input needs *three* leads, but a scope probe only needs
    Last time I looked, test equipment is exempt, and you'd have to get
    caught anyhow.

  12. John  Larkin

    John Larkin Guest

    Just the capacitance of the enclosure to the universe will be a lot
    more than that. And the ground side will squirt massive ammounts of
    hum and noise into the circuit under test.

  13. Joerg

    Joerg Guest

    Hello Stanislaw,
    ECG inputs are usually fully isolated. Have to be.
  14. Joerg

    Joerg Guest

    Hello John,
    You could also use a couple of tiny iso transformers. One wideband for
    the signal, the other to drive the DC restore.
  15. oops, you're right, a one-meter sphere is around 60pf, the Earth is
    around 700uF, so a Tek scope is going to be 20-40pF I'd guess.

    So jsut add another kludge-- sense the source voltage, buffer it, and
    feed it to a screen cage around the scope. That will take care of the
    capacitance and the noise. Watch the fingers though.
  16. John Larkin

    John Larkin Guest

    A common-mode choke/balun can help a lot, or wrap the probe lead
    around a ferrite toroid a few times. That at least improves the hf
    cmrr a bit.

  17. Jim Yanik

    Jim Yanik Guest

    If you just want a floating scope input,get a TEK A6902.
    Or order the service manual for the P52XX and build one yourself the way
    you want.
  18. Nothing to do with measuring the signal and CMRR. Safety = 50 uA (
    that's my keyboard spelling micro) at line frequency can (does NOT have
    to ) throw the heart into pachanga dance (medically called fibrillation).

    This part of medical electronics many times IS the major time consuming
    part of designing an intensive care room equipment.
    There are NO excuses.

    Have fun

    Slack user from Ulladulla.
  19. Rich Grise

    Rich Grise Guest

    This rhymes with "sludge". It's KLOOGE! >:-[
    The times I've needed differential probing, I just used the 454, with two
    of the same model of probes, and put the vertical on A-B. :)

  20. Keith

    Keith Guest

    That works for *really* slow signals where CMRR isn't an issue.
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