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Neophyte Question: measuring open circuit voltages

Discussion in 'Electronic Basics' started by [email protected], Dec 15, 2005.

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  1. Guest


    I'm trying to measure the voltage induced by a peizoelectric cable
    (applying pressure to cable creates voltage/current on cable). But
    when I measure the voltage across the cable I get a "background"
    signal that looks like an exponentially decaying voltage, like a large
    capicitance discharging. I can measure the real voltage across the
    cable, it appears on top of the decaying voltage, but soon the voltage
    reaches what I assume is the max low value for my board (National
    Instruments, BNC 2120, floating point measurement), -10 volts. I can
    get the voltage to go back to zero by touching the leads but this just
    sets the process in motion again. This decaying voltage characteristic
    is also present when nothing is connected to the leads, which makes
    sense, hence my question about measuring open circuits. Needless to say
    I would like the voltage of an open circuit, or accross my cable to be
    0. How can I make this happen?

    Thanks in Advance.
  2. The Phantom

    The Phantom Guest

    Your NI board input circuitry has a bias current, probably in the nanoamp
    region, which is charging up the cable. You could try injecting a current from
    a stable DC voltage of opposite polarity through a *very* high value resistor,
    adjusting the voltage and resistor until the voltage on the cable is nearly
    constant. Then flex the cable and take your measurements quick like a bunny.

    Or, get one of these:

    and use it as a buffer to get the input current down to the femtoamp range. You
    then momentarily short the leads to reset the voltage to zero, and the bias
    current of the LMC6001 will charge the cable so slowly that you should be able
    to get your measurements.
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