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Voltage signal (-0.40mV ~ 7.60 mV) to NI card and LV

Discussion in 'Electronic Design' started by Lathe_Biosas, Oct 2, 2005.

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  1. Lathe_Biosas

    Lathe_Biosas Guest

    Hi

    I'm trying to plot a voltage signal (-0.40 mV ~ 7.60 mV) on LabView
    unfortunatelly I haven't achieved reading the signal with mV
    resolution, my conclusion is that the signal needs to be amplified.

    Could anyone please tell me if amplifing the signal would help? If yes
    how can it be amplified?

    I have an Analog Devices AD595 (Monolithic Thermocouple Amplifier)

    http://www.analog.com/UploadedFiles/Data_Sheets/421725987AD594_5_c.pdf

    and I was wondering if it would help


    Any help or hints would be kindly appreciated
    Best Regards
     
  2. The AD622 works for me in situations like this.

    Cheers
     
  3. LabView is software, not hardware. From your title I guess you're using a
    National Instruments data acquisition card. They make quite a few models.
    Which one are you using?

    But anyway, yes, amplifying it would probably help; alternatively, you need
    data acquisition hardware with appropriate resolution (for instance, a
    digital voltmeter with a mV scale and a computer output of some sort).

    Accurately amplifying a DC signal that small is not trivial. Some things
    you'll need to specify are:

    - what resolution do you need? E.g., 10uV, 1nV, ...?

    - how much drift, over time and temperature, is acceptable? (Are you
    trying to measure over a timespan of milliseconds or days?)

    - what speed do you need? E.g., is this a slowly-varying DC signal, or a
    1MHz waveform, or ...?

    - what is the output impedance of the signal source? (Alternately, what
    input impedance do you need from your amplifier?)


    Are you measuring the output of a thermocouple? If not, then probably not.

    But anyway, achieving fractional-mV resolution in a DC amplifier depends as
    much on good circuit design and layout skills as on the parts you use, and
    just having the right IC (even if it were, which it probably isn't) is not
    sufficient.
     
  4. Lathe_Biosas

    Lathe_Biosas Guest

    LabView 6.1 with a PCI-6024E
    A Fluke 192B Scopemeter was used with success. Now the goal is to see
    a waveform in Labview
    Steps of .70 mVols aprox but it doesn't need to be extremly accurate.
    It could be nice to graph on Labview using for example a waveform.
    Going from aprox. -0.40 to 7.60 mV takes more or less 5 minutes.
    As much as I know it is a slow varying DC signal
    Unfortunatelly I don't know that.
    Yes, I'm measuring the output of a thermocouple.
    Thank you very much for your answer, so what would you recomend me to
    do?

    Best Regards
     
  5. Lathe_Biosas

    Lathe_Biosas Guest

    Hi,

    Thanks for your answer, I will look for the AD622 (cool device), if I
    don't find it I will try with a 741

    Regards
     
  6. The 622 is an instrumentation amp, not an OPamp like the 741. It has very good offset voltage typ 125uv.

    You should amplify the signal to ~4v for the NI 6024. I believe this is a 16bit card. For the +/-5 volt range the resolution is
    150uv. Which is adequate.

    Cheers
     
  7. So, the simplest and best thing you could do would be to get a DAQ card with
    a thermocouple input, such as the NI USB-9211, which they list at $395, lead
    time <1wk. I don't know your budget, but I know that building a homebrew
    thermocouple amp, if you've not done it before, is going to cost you at
    least $50 in parts plus at least 2 days of work, and you won't end up with
    anywhere near the accuracy, nor will you end up with something resellable;
    so you can do the cost-benefit analysis yourself. (To the naysayers: yes,
    if you've got some prior experience and a well-stocked junk drawer, you
    could do better; but if you haven't done it before, then no.)

    Barring that, given that you've got the thermocouple amp chip, starting with
    its app notes seems like a reasonable approach. If they make no sense to
    you, that's informational.
     
  8. Don't bother with a 741.

    Look at the DC offset voltage spec.
     
  9. Pooh Bear

    Pooh Bear Guest

    Forget the 741.

    If you just need reasonable DC precision the LM11 may fit the bill.

    Graham
     
  10. Lathe_Biosas

    Lathe_Biosas Guest

    Hi

    I have an LM311, would it help?

    Regards
     
  11. Not at all.



    Best regards,
    Spehro Pefhany
     
  12. Here I must respectfully disagree with Spehro. The LM311 is just what you
    need. Put it in a box - ideally, you might want to solder it to a printed
    circuit board, and then mount the printed circuit board in a box, so that it
    doesn't rattle. Also put some audio connectors in the box, and wire the
    input connectors to the output connectors. Now, sell the box to an
    audiophile, for $395 plus tax and shipping. Use the proceeds to buy a
    thermocouple card to run LabView with.
     
  13. Jim Thompson

    Jim Thompson Guest

    Sno-o-o-ort :)

    ...Jim Thompson
     
  14. Oh, my GOD! I laugh at the same stuff as Thompson? Will somebody please
    euthanize me?

    ;-D
    Rich

    (By the way, this is a joke.)
     
  15. Fred Bloggs

    Fred Bloggs Guest

    Could anyone please tell me if amplifing the signal would help?

    Not as much as reading the specifications- the 6024E can be programmed
    for FS of +/- 50mV with 0.008mV resolution and 0.106mV absolute accuracy
    using 100 point averaging.
     
  16. Lathe_Biosas

    Lathe_Biosas Guest

    Hi,

    I choose the AD620 but I don't know if I wire it right

    -IN and +IN are connected to the Thermocouple
    -Vs and +Vs to power supply -12 and +12 Volts
    REF is connected to ground
    OUT directly to the voltmeter

    The gain was set to aprox. 1000 with a 47 Ohm resistance

    Is that ok? Unfortunatelly with the power supply -Vs=-5 Volts and +Vs=5
    Volts the output
    doesn't represent a gain of 1000.

    Any recomendation?
    Best Regards
     
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