lm35 with ni daq problem

Discussion in 'The Projects Forum' started by mah, Feb 6, 2014.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
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    i use usb6008 daq to acuire signal from lm35 but the output of the sensor is affected when connecting the daq on its output , the out of the sensor is .45 volt when connecting the daq it reads the output less than the real output by .04 and this value decreased when the output increased . should i use optocoupler or what do you think ?
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    998
    I don't know anything about your DAQ, but my lm35 is connected directly to my ADC with no issue. You need to check the input impeadance spefification of your DAQ. It should be several megohms. If that is the case, then you have a setup issue. It that's not the case, then you need a high input impeadance unity gain amplifier between your lm35 and DAQ.
     
  3. TANDBERGEREN

    Member

    Jan 20, 2014
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    If any problems with iimpedance, it is quite simple to use nearly any OP-amp as a voltage follower to fix that problem.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You need a negative supply for it to be good to 0C. See fig. 1 &2 in the datasheet.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    If I remember correct the input impedance of the 6008 is quite(very) low for a DAQ device. But before I dive into this case post your schematic
     
  6. TANDBERGEREN

    Member

    Jan 20, 2014
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    On page 8 in the datasheet (National Semiconductor November 2000) there is two good examples of connecting it directly to a ADC.
    But there is no problem using the datasheet as a direct source for Your design.
     
  7. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    the output is directly connected to the ADC , in my design i used 10 Kohm in series with the lm35 and a 1 μf capacitor as RC damper
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    This is the input stage of your DAQ unit. Also shown in the manual page 11 http://www.ni.com/pdf/manuals/371303m.pdf As we can see it is a very poor design. In most cases an opamp buffer stage before the input will be required
    [​IMG]
     
  9. TANDBERGEREN

    Member

    Jan 20, 2014
    74
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    Not a good design.

    Use a LM101 or similar.
    connect the output of the LM35 to the non-inverting input
    Connect the inverting input to the output.
    Connect the output of the OP-amp to the DAC input.

    Any damping is not necessary.
    But if You find it so anyway, then put the damping on the OP-Amp input.

    Have a look here: http://pdf1.alldatasheet.com/datasheet-pdf/view/8594/NSC/LM101.html
     
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