Coaxial/triaxial Cable Impedance matching

Discussion in 'The Projects Forum' started by raj1391982, Nov 29, 2007.

  1. raj1391982

    Thread Starter New Member

    Dec 20, 2006
    4
    0
    Hii.....
    i m stuck somwhr in my project, and ur valuable opinions and suggestions are invited.....

    Brief about Project:
    I hav to acquire sensor data from a system which is terminated in 10MOhms. i have to use a cable (may be triaxial or coaxial cable) to connect it to my signal conditioning card.
    a. signal level is +/- 100V and +/- 100mV
    b. frequency of signal is 1kHz to 100KHz
    c. magnetic field of 20Gauss arround the system giving di/dt= 10KAmp/sec

    Issue:
    i have to match the impedance of the sensor card impedance, cable and my signal conditioning card impedance.
    Que 1. do length of the cable make any difference in impedance matching.
    Que 2. what i hv to do for impedance matching?
    Que 3. does magnetic field of 20Gauss affect my system? how much? how can i avoid it?


    please let me know if any other details required.....
    Thanx.....
    Cheers...
     
  2. 3BwEH

    Active Member

    Feb 25, 2007
    33
    0
    of course the length of the coaxial cable makes a difference actually its all about the length u are using a quarter wave lenght but anyways i cant see you matching at that frequency because you need an awful long cable to match ur impedances anyways
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You don't have to worry about impedance matching - just driving the coax capacitance. Use a buffer op amp for that job. Something like the OPA134 with high input impedance and the ability to drive capacitive loads.

    Triax is an expensive and hard-to-connect mess that the buffer amp should make very unnecessary.

    No experience with a field like that - I would stay away from a steel enclosure to avoid induced currents. Use a cast aluminum box for shielding, though. That 10 meg circuit is going to be prone to see interference.
     
  4. raj1391982

    Thread Starter New Member

    Dec 20, 2006
    4
    0
    thanx for all ur valuable suggestions........
    i m bit confused......
    sensor ----------1-------> cable ----------2---------> Signal conditioning card
    10MOhmtermination ---1--> 50Ohm - 75Ohms ----2----> resistor divider to attenuate ------>isolation amplifier
    so whr i need to keep the opamp? do i need to match sensor termination and cable impedance? or to match cable impdeance and signal card impedance?

    important point: signal level is +/- 100V, so can any opamp take this much of signal input?


    i think i need to keep matching network(resistor network) between sensor termination and cable..... also need to match cable impedance and signal conditioning card input impedance...... most probably i wud go for resistor network for impedance matching....
     
  5. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Can you go to fiber optic cabling? Then you can avoid issues with the magnetic field and impedance matching (if it is necessary). How long of a run is it?

    There are some opamp configurations that can do 100V -- are pushing that through the 10Mohms? The buffer amp comes before the long cable run.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I skipped over the +/100 volts part. But, are the two signal levels related? If the millivolt level signal is the scaled-down counterpart to the 100 volts one, then my answer is relevant.

    If you do need to push 100 volts down the coax, what kind of device were you going to use at the other end?
     
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