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*Tricky* current measurement.

Discussion in 'Electronic Design' started by Fred Bartoli, Feb 27, 2007.

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  1. Fred Bartoli

    Fred Bartoli Guest

    I've some requirements for some challenging voltage measurement (I know,
    subject said current, but see later). Namely:
    - up to+/-4kV pk WRT ground
    - % order accuracy up to 50kHz
    - duty cycle max 10%

    'til now, no pb.

    Then the fun part:
    - 80dB CMRR up to 100kHz
    - 60dB CMRR up to 10MHz (yes!)
    - the signal can go down to 4V and yet 1% accuracy is required (which is
    an overall 16b accuracy)

    No pb with power drawn by the probe but they don't want maintenance,
    periodic cal and so on...

    We are politely (well not so politely) asked to do this à la 2x
    compensated dividers then diff amplifiers. Yeah...

    OK. Now I have to backup that crap with a working solution, or as near
    as possible to the specs.

    My first thought was a load resistor (told you it was current
    measurement:) and two current shielded xfrmrs (two to reject the CM
    currents from load to shield) for the BW, and a servoed DC-LF
    compensating current, all that almost working like the Tek probe.

    Unfortunately, looking closer, the LF-DC part isn't going to be that
    easy (mainly Hall sensor noise, offset and low sensibility...) :
    - the load can't go much below 100K (10W on average) giving a 40mA-ish
    pk current (plus the needed HF zero).
    - Hall sensors seems to be somewhat noisy
    - can't use multi turn primary, due to the then uncontrollable
    parasitics (still CMRR)

    Thought about fluxgate but the switching frequency will be well into the
    signal BW, so it's probably not OK. Plus I can't think of a simple way
    to implement a gate suitable to the design (100, maybe 200 units made).

    Any clever idea welcomed.
  2. MK

    MK Guest

    "Fred Bartoli"
    Hello Fred,

    This will only help a bit because these people make high current sensors: Ultrastab 867-200I transducer.

    200A full scale but errors and noise in the 10 ppm range .

    If you wind more turns round it the sensitivity goes up but new errors may
    come in. If it worked with 100 turns it would be 2 A full scale so errors in
    the 20uA region which is 1% of 2mA which would be fine if you could afford
    2A full scale current (only 8kW !! :)
    With your 40mA at 4kV max you are looking at an equivalent voltage error of
    2V which is about 50 times outside your budget.

    However you may get some ideas from what they can achieve and the way they
    do it - you just need a 4A full scale device and you're there.

    Good luck.

    Michael Kellett
  3. Tim Williams

    Tim Williams Guest

    What's wrong with a current transformer? DC capacity?

    Maybe you could mix a limited-bandwidth (less noise!) Hall effect signal
    with the transformer's signal output?

  4. Fred Bartoli

    Fred Bartoli Guest

    Tim Williams a écrit :
    A current transformer alone won't do. You need two, to compensate for
    the sensed wire parasitic current to shield/gnd. And then you still have
    to be very cautious: with a total 10p across the 100K load, you still
    need less than 10fF parasitics imbalance to achieve the 60dB CMRR.
    Can't go much over 10pF there because dV/dt are 20-50V/ns which is
    already 500mA. Still, 100pF requires less than 0.1pF imbalance, easing
    things a bit, but at a 5A switching transient cost.

    Otherwise it's more or less what I was thinking about, but in a better
    way: if you just mux then you have some gain accuracy pb at crossover
    and most probably some non linearity due to the core hysteresis.
    Using FB to close a current compensating loop around the core takes care
    of most of this.
    But Hall sensors are... noisy. Just think:
    A 20mm dia toroid, mur=10K, will only give 8mT flux density at my
    4kV/40mA peak current.
    But 1% of 60dB down is 100dB, so I want less than 80nT rms noise in the
    bandwidth in which the Hall sensor will take over, not accounting for
    the higher frequencies amplifier part.
    And CT, even shorted, have limited LF response so I can't make the Hall
    sensor branch crossover too low.

    Oh, and I forgot to tell: 10MHz -3dB response, but that part is POC with
  5. Chris Jones

    Chris Jones Guest

    Why not use an amplifier and ADC, all floating at the high voltage, and then
    clock out the output into a fibre-optic link. The ADC and fibre optic
    transmitter (and probably a microcontroller) would need some power but
    whether that is difficult depends on constraints of cost etc. (e.g. solar
    cells + bank of infrared LEDs can transfer a few milliwatts, or for more
    power a carefully constructed transformer.)

  6. jasen

    jasen Guest


    sounds like Massive Prong.
    except for the above, I'm all out.
  7. Terry Given

    Terry Given Guest


  8. joseph2k

    joseph2k Guest

    Perhaps a comapany called "Ion Physics" has some CT type sensors that can do
    something close to what you want.
  9. Robert Baer

    Robert Baer Guest

    Some work at SLAC was done in that regard years ago; i forget even
    the order of magniture that they were able to achieve, but suffice to
    say they pushed existing technology a fair amount.
    Application? Measure the (pulsed) beam current.
  10. The 16-bit measurement rate would be under, say, 200kHz? Why
    does the CMRR spec go to 10MHz, or are you saying that's an
    overall spec that can be met in part with a sharp signal filter?
  11. Fred Bartoli

    Fred Bartoli Guest

    Winfield Hill a écrit :
    My bet is that the spec is coming from a nice anti-cooperative working

    A (probably unexpectedly) sold a rough prototype, with as much
    carelessness designed in as possible (IOW a "demonstrator"), to B under
    the pressure of C (if you B still want to sell us in future, go have a
    look at what A do) without clear understanding of all the implications.
    Then D got a contract to ('just') industrialize the product.
    Now that A sold its s..t and that B almost don't understand how it works
    (I mean the fine important details), specs are flying over in an attempt
    to making all that falling in working order and also probably to cover
    one's ass as much as possible.
    And as A and B almost don't speak to each other anymore, and A refuses
    to speak to D, and B pretend to not being able to disclose some
    information, then having the real info is, hem, well hem...
    And this is taking interesting proportions.

    So, despite that I think the sought CMRR is totally irrelevant we have
    to comply to what we are given.
    So yep, the CMRR has to be as asked, not a by product of some after

    For the pb I think I've now come up with a nice working solution but it
    probably won't see the light since B has now decided to engaged into the
    2 x compensated dividers way and some other 'interesting' problems are
    pointing their nose.
  12. Sheesh!
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