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I have a question about voltage-to-current conversion.

Discussion in 'Electronic Design' started by Victor hyppolito, Jun 21, 2006.

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  1. I have a question about voltage-to-current conversion. I have a small
    signal(0-10mV) that is amplified (0-5V). I have to send it to a
    computer that is located 10 meters away. The question is: is it better
    to convert the voltage signal to current? If so, why? I have this
    IC(XTR106) that is a voltage-current converter but I´m having some
    problems with it, so, if it were not necessary to use this technique I
    would just send it as a voltage signal instead.
    Any suggestions?
    Thanks a lot!
  2. Guest

    current signals are good for long runs where voltage drop across the
    conductor becomes a problem. 10m with a properly gauged conductor a
    0-5V signal should be ok
  3. John B

    John B Guest

    Victor hyppolito scrobe on the papyrus:
    Why not convert it to a number with an ADC and then send an ASCII
    string over a serial link? That way you can send it as far as you want.
  4. The choice has a lot to do with degradation of the signal as seen by the
    computer. This depends on the source and load impedances, noise sources,
    and choice of conductors. A 0-5V signal in a two conductor shielded cable
    should be fine, especially if the conductors are internally twisted, and
    the load impedance is high. Another problem with voltage is when there is a
    ground loop from the computer to the sensor, but this is not a problem if
    differential means are used.

    Current signals (typically 4-20 mA) are used in industrial monitors and
    controls where the load can have electronics which are powered from loop
    current, and it adjusts the load current which is read by the sender. This
    allows power and signal to share a single pair of wires, and current is not
    affected by wire resistance.

  5. Ban

    Ban Guest

    But how do you input this signal into the computer? Do you have a data
    aquisition card or do you use your soundcard for it. And what kind of signal
    is it and how do you amplify it? Tell a bit more about what sensor you are
    using, maybe it isn't even necessary to amplify or causes more errors. A
    10mV signal is pretty strong and if it is of low impedance it can usually be
    connected to a acquisition card directly.
  6. Joerg

    Joerg Guest

    Hello Victor,

    If the computer does not have a good AD converter built in but does have
    a sound card, I'd say no. I would convert to a frequency. Reasons: The
    game port AD conversion is often not that good (and none of mine have a
    game port to begin with). With the sound card you can use a simple FFT
    routine to determine the incoming frequency. There is an abundance of
    such SW as freeware or shareware.

    A good voltage to frequency converter chip is National's LM331.
  7. Hello Ban

    I´m working with strain gauges. My goal is to amplify the output
    signal of a wheatstone bridge(0-10mV) and send it to another room(10
    meters away). Afterwards, I would input it in an aquisition card
    (basically a adc converter and a PIC). Thats why I think i need to
    amplify it and maybe send it as a current signal. The data will go to
    the computer via rs232 using an IC max232 to convert TTL to -12, 12V.
    Thanks for the reply!
    Victor D'ippolito

    Ban escreveu:
  8. Jim Thompson

    Jim Thompson Guest


    Have you considered an AC excitation?

    ...Jim Thompson
  9. Joerg

    Joerg Guest

    Hello Jim,
    Preferably at a frequency outside the harmonics of 60Hz or in Victor's
    case probably 50Hz.
  10. John B

    John B Guest

    Victor hyppolito scrobe on the papyrus:
    ... <snip> ..

    I get bad karma about this idea. Why have your ADC/PIC card 10 metres
    away from the signal source? It should be as near to the strain gauges
    as possible. The RS232 signal can travel considerably more than 10
    metres with minimal degredation.
  11. a) 10mV IS very strong signal as far as strain gauges are concerned.
    b) in your place would just use 2 pair shielded cable and "Damn the
    c) The conversion to current is very convenient when working in very
    noisy enviroment or working on bodies where weight is of concern.
    The weight of wires and their connectors on properly "telemetered" plane
    or missile is to be considered. As long as the connection exist you have
    a current flow. In lab conditions, not worth the effort.

    Have fun

    Slack user from Ulladulla.
  12. Hello Jim

    I haven't considered AC excitation... Actually I don't know how to use
    AC excitation. What signal should a use? A sine wave, square wave? Do I
    use RMS valures?
    Sorry about my ignorance!
    Thank you for the reply!
    Victor D'ippolito
  13. Jim Thompson

    Jim Thompson Guest

    Just drive your bridge with a sine wave instead of DC.

    I've always been fond of sine wave excitation since it makes getting
    rid of OpAmp offset voltages a snap.

    ...Jim Thompson
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