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measuring 0 - 30mV — Parallax Forums

measuring 0 - 30mV

mikeweezermikeweezer Posts: 27
edited 2005-10-28 18:30 in General Discussion
Hello again all,

It looks like I am going to purchase a device that outputs between 0 - 30 millivolts depending upon what it is sensing. Does anyone know a good device / or circuit that I could use to interface this with either my Stamp or a PC?

Thanks,
Mike

Comments

  • stamptrolstamptrol Posts: 1,731
    edited 2005-10-14 16:11
    Mike,

    Build yourself a non-inverting op-amp with a gain of about 100. Then you will have 0 - 3 volts to work with. That will work nicely with an LTC1298 a/d converter which the Stamp can read easily. For more range, add another stage of amp or bump the gain to 150 - 175.

    For circuits, just Google non-inverting op-amp.

    Tom
  • steve_bsteve_b Posts: 1,563
    edited 2005-10-14 17:17
    Sounds like you've already spotted teh measuring device andyou're asking how to interface it.
    Helps to know what the device is you're trying to interface....

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    ·

    Steve

    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • nick bernardnick bernard Posts: 329
    edited 2005-10-14 17:38
    this is a basic example of an variable gain amp, good for conditioning and tuning dc levels. the follower at the end of the amp provides low output impedance to the adc.
    roxon
    nick

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    bmp
    bmp
    jank.bmp
    38K
  • mikeweezermikeweezer Posts: 27
    edited 2005-10-14 18:58
    sorry about that...
    the device is a Load Cell from Omega Engineering

    http://www.omega.com/ppt/pptsc.asp?ref=LC105

    I am looking for a simple way of telling how hard people are pulling on a rope...

    Thanks
  • KenMKenM Posts: 657
    edited 2005-10-14 19:39
    AD620N

    http://www.analog.com/en/prod/0%2C2877%2CAD620%2C00.html

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    Ken

    Post Edited (KenM) : 10/14/2005 7:42:55 PM GMT
  • SPENCESPENCE Posts: 204
    edited 2005-10-14 20:03
    BRAINSTORM.! I HAVE BEEN TOLD THAT TAyLOR WEIGHT SCALES USE LOAD CELLS. THEY MAY HAVE SEVERAL CELLS IN EACH UNIT.

    PROBABLY WOULD BE MUCH CHEAPER THAN OMEGA AND STILL FIT THE CAPACITY YOU NEED.

    73
    SPENCE
    K4KEP

    Post Edited (SPENCE) : 10/14/2005 9:17:58 PM GMT
  • ChrisPChrisP Posts: 136
    edited 2005-10-15 14:34
    Mike,

    I'm very novice at electronics but loads cells are my specialty, the AD620 chip that Ken suggested is ideal and widely used commercialy to amplify the signal from a load cell to a level that is more easily sampled. One thing your likely to run into is 0 offest of the load cell. This is easily cancelled out with the 620 by applying an equal but opposite voltage to its offset pin through the use of a voltage divider or dynamicaly through PWM, D/A converter etc. For simplicity a voltage divider works fine. If your not looking for precise readings then just ignore the offset correction, it will still give repeatable readings, just not actually accurate. Also in my little bit of experience with signal conditioning its been a lot more workable to run the load cell and 620 from the same power supply, either excite the load cell at 5 volts or run your 620 from the same power source that your load cell is excited with, otherwise your likely to see the reading float for no apparent reason (which is probably very apparent to some here, just not me). $295 for the level of precision listed seems very reasonable to me but most of what I work with are torque·load cells·which are quite a bit more expensive than linear force.·I doubt your going to have better luck with scales since for reasons I wont go into here they take quite a bit more development for static load cells rather than dynamic. One quirk to mention that you may run into is that load cells under opposite load, in this case compression rather than tension tend to reverse the polarity of their output. This doesnt bother the 620, however depending on your circuit it could end up driving it into the power supply rail if someone were to apply a compression load instead of tension.



    Hope the post was some help
  • KenMKenM Posts: 657
    edited 2005-10-16 15:51
    nick bernard said...
    this is a basic example of an variable gain amp, good for conditioning and tuning dc levels.



    And very poor bandwidth as the you increase the gain.

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    Ken

    Post Edited (KenM) : 10/16/2005 3:56:41 PM GMT
  • nick bernardnick bernard Posts: 329
    edited 2005-10-18 14:58
    obviously, thats why i use two stages, but at very low frequencies that doesnt matter.

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    engineer, fireman, bowler, father, WoW addict [noparse];)[/noparse]
  • KenMKenM Posts: 657
    edited 2005-10-19 02:28
    First.....I don't know how to state this without sounding like I am challenging you....I am not....

    I thought that the bandwidth is a function of overall gain, whether it is done in one stage, or X number of stages. If the overall gain is 100, you will essentially have the same loss in bandwidth if that 100 gain is achieved with one stage, or two stages etc...?

    And as you stated, yes,·at very low frequencies that doesnt matter.

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    Ken
  • nick bernardnick bernard Posts: 329
    edited 2005-10-24 13:59
    well i thought the gain bandwith product applied to the op amp not to the ic. i looked it up in a college text and didnt find any info that suggest otherwise. but hey, it isnt a very good book. inanycase thats how i was taught how to get around the gain bandwith product in my metrology course.

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  • metron9metron9 Posts: 1,100
    edited 2005-10-28 18:30
    Link to optional sensors for your project

    http://www.meas-spec.com/myMSI/default/sensorSelect.asp
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