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    Best isolated or differential scopes/probes

    Like others I'm sure, we constantly run into the limitations of our differential probes when looking at high CM slew signals like high voltage high side gate drive.

    What equipment do other people like for these applications? Are there better probes than what I'm aware of?


    Things I'm aware of:

    Example differential probe:
    https://www.tek.com/datasheet/high-v...rential-probes
    DC: > -80 dB
    100 kHz: > -60 dB
    3.2 MHz: > -30 dB
    100 MHz: > -26 dB

    Tek's very expensive all optical isolated probe:
    https://www.tek.com/isolated-measurement-systems

    Isolated Scopes:
    https://cleverscope.com/ (~$10k, 100dB CMMR@50 MHz)
    https://int.siglent.com/products-overview/shs1000/ (~$2k, >100:1 CMMR@50MHz)
    Tek THS3000 (~$5k, can't find CMMR)

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    Re: Best isolated or differential scopes/probes

    For the said measurement application (high side gate voltage), you need really high CMRR and a probe design that doesn't send common mode currents down the probe cable.

    The Tek optically isolated probe system is currently unrivaled in this regard.

    Most differential high voltage probes have too low CMRR. Even a probe with relative good SMRR specification like Agilent N2804A (300 MHz 100:1, 1000 V common mode voltage), see below CMRR curve, seems to have difficulties with high common mode dV/dt according to my tests, apparently a nonlinear effect that isn't reproduced in the small signal CMRR.

    Click image for larger version. 

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    I have also doubts if the impressive input spec of isolated cleverscope channels can actually work with usual single ended probes. Maybe someone has experiences with this device?


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    Re: Best isolated or differential scopes/probes

    Thanks. So do you have a preferred differential probe or is your impression that they're all similar?

    There is an EEVBlog video discussing the design of the cleverscope and showing it off in a gate drive application
    https://www.youtube.com/watch?v=I7ppDNLlEL4



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    Re: Best isolated or differential scopes/probes

    So do you have a preferred differential probe or is your impression that they're all similar?
    I haven't yet found a differential probe that is suitable for measuring high side gate voltage without huge common mode crosstalk. I didn't yet check the PMK "BumbleBee" 400 MHz probe, but from CMRR specification it isn't better, just faster. It's s probably well suited to measure drain-source or bus voltage in SiC switchers. Presently I'm using 100 MHz differential probes like Testec SI-9110 or Keysight N2790A for the high voltage range, or single ended passive probe where appropriate.

    I reviewed a cleverscope application note which looks promising:
    https://cleverscope.com/downloads/fi...asurements.pdf

    I'm also investigating simple analog fiber transmission channels for high side gate measurements, 100 MHz seems feasible, linearity and SNR is however limited.


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    Re: Best isolated or differential scopes/probes

    Bumblebee is 50dB at 3.3Mhz
    Testec SI-9110 is 60dB at 1MHz
    Keysight N2790A is 50dB at 1MHz

    Those are all better than the Tek probe I linked above so that's useful.

    A couple others:
    Pico TA042 50db at 1Mhz
    Lecroy HVD3000A 65db@1Mhz (the best conventional differential probe here)


    And another optically isolated one I didn't know about for about $4000:
    Lecroy HVFO103 Series is 120db at 1MHz
    http://cdn.teledynelecroy.com/files/...-datasheet.pdf


    While on the subject of cleverscope here is a neat document describing the design and their design challenges:
    https://www.eevblog.com/files/CleverscopeSep2018.pdf


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    Re: Best isolated or differential scopes/probes

    Cleverscope isolated probe - excellent ...



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    Re: Best isolated or differential scopes/probes

    I take it you have one? Can you say anything more about it than that?



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    Re: Best isolated or differential scopes/probes

    I can say it's designed in New Zealand - and very possibly the best on the market at the moment - we look at a lot of high side gate drives so ...



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    Re: Best isolated or differential scopes/probes

    I wish there were more diff probes for general purpose low voltage applications. Nowadays I use differential signalling whenever possible analog circuits (especially interfacing to ADCs), and the high voltage diff probes are no good for such low level signals. I wish there was something like the old tektronix p6046, but not such a PITA to calibrate and use. Maybe the cleverscope is the answer to that as well? How is their DC offset?



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    Re: Best isolated or differential scopes/probes

    I agree. One of the first things you have to explain to a new engineer is probe grounding and most of the time it doesn't sink in until they blow something up. And for experienced engineers it's plenty easy to make a mistake - like forget a ground loop formed through the USB debugger.

    Differential or isolated probes make life much easier.

    Seems like scope manufacturers might want to consider cheaper differential/isolated options even if it means lower performance.



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