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Discussion in 'Electronic Design' started by Mark, Dec 3, 2003.

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  1. Mark

    Mark Guest

    I'm using a LEM current transducer to measure pulsed currents. The
    current sensor produces a current output which I feed to a resistor
    producing a voltage that represents the current flow. The voltage feeds
    an opamp for offset correction and buffering. The sensor has good
    accuracy (+/- 0.5%), but suffers from an initial offset
    (+/- 0.15mA) and worse, a thermal induced offset (+/- 0.35 mA over temp).

    Since I know when the current pulses are coming, I want to to force the
    offset to zero before taking a measurement. I've seen circuits to do
    this using the following techniques:

    1. An A/D followed by a D/A. With zero current input, the offset is
    driven to zero, and the offset correction is latched in the D/A until
    the next "calibration" cycle.

    2. A/D and a digital pot controlled by a processor, works similiar as
    above.

    What are the merits of either method. I prefer not to have a processor
    involved, but do have access to FPGA resources.

    Is ther a simple way to do this?

    Mark
     
  2. Jim Thompson

    Jim Thompson Guest

    What sort of pulses... amplitude, width, rate?

    A simple DC restorer might work under some circumstances.

    ...Jim Thompson
     
  3. John Larkin

    John Larkin Guest


    in--------c-------+----------out
    |
    switch
    |
    gnd


    Just keep the switch closed when you know there's no pulse. A fet or
    an CMOS analog switch thingie will work.

    John
     
  4. Genome

    Genome Guest

    Bit of shit in the schematics newsgroup

    BLART
     
  5. Genome

    Genome Guest

    Serious? You might like to read that one again..... Oh, I'm sure you knew
    anyway.

    Has DM learnt about transient thermal impedance yet?

    BLURP
     
  6. Luhan Monat

    Luhan Monat Guest

    Mark,

    Since you are incorporating a processor here, make 2 a/d readings: one
    just before and one during the pulse. Then subtract the first from the
    second.
     
  7. John Fields

    John Fields Guest

     
  8. scada

    scada Guest

    Usually the Hall Current Transducers are matched with a signal conditioner.
    The conditioner handles excitation current, and offset balancing. A visit to
    LEM's web site might be helpful.
     
  9. John Larkin

    John Larkin Guest

    Er, eh, wot? Did I miss something?

    switch closed when no pulse

    switch open when we have a pulse

    looks OK to me. OP sez he knows when it will hit.
    Geez, don't confuse the poor slob; he's having enough trouble with the
    steady-state stuff.

    John
     
  10. There is a single chip solution for this sort of problem.

    http://cache.national.com/ds/LF/LF198.pdf
     
  11. The old way of doing this would be a zero-crossing
    comparator on the output volts, setting the direction
    of an up/down counter, which drives a DAC. With the
    counter clock running, the system auto-zeroes, with a
    hunt of +/- 1-lsb around zero. The size of 1-lsb is
    determined by the hysteresis of the comparator and
    the resolution of the DAC. Stop the counter clock to
    Hold the auto-zeroed compensating offset.

    To avoid the need for a bipolar DAC put the comparator
    ref input on a small positive offset, which is taken out
    with an identical negative offset downstream.

    For your app it looks like the DAC needs the equivalent
    of at least 2mA full scale (+/- 1mA), with a resolution
    of (say) better than 0.1mA. An 8 bit counter+DAC with
    5mA full scale (+/- 2.5mA) would have a resolution of
    0.02mA..... probably more than enough.
     
  12. Mark

    Mark Guest

    Yes, this was one of my thoughts also. Thanks,

    Mark
     
  13. Mark

    Mark Guest

    Another foos suggestion! Thanks,

    Mark
     
  14. Mark

    Mark Guest

    I don't see how this helps, The offset is inherent in the sensor, so
    when I open the switch I still get measurement + offset.

    Mark
     
  15. Mark

    Mark Guest

  16. Fred Bloggs

    Fred Bloggs Guest

    You're right- this Larkin is off in left field most of the time- in lala
    land of egomaniacal euphoria over delusion of being God's gift to
    engineering- when he's not acting the shameless NG toady that is...
     
  17. Fred Bloggs

    Fred Bloggs Guest

    Then again if that "c" means capacitor-he is actually trying to
    communicate an age-old technique of charging the cap to a Vos-from
    left-to-right so your digitizer or whatever then reads
    Vpulse+Vos-Vos=Vpulse during the measurement, so you would put your
    burden resistor to the left of "c" in the diagram, drop it from "in" to
    GND, to convert current to voltage, and select "c" to pass Vpulse with
    negligible droop.
     
  18. John Larkin

    John Larkin Guest


    Welcome to la-la land. Can I get you a smoothie?

    John
     
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