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analog output question

Discussion in 'Electronic Design' started by Jim, Jan 21, 2008.

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  1. Jim

    Jim Guest

    I have an Omega OS102 ir thermometer that I am using in a project to record
    temps continuously. My data logger has 0-5 volt inputs at 12 bit
    resolution.

    The Omega unit has an analog output of 1 mv per degree C. I am doing all my
    recording in the 5-37 degree C range, and it would be very useful if I was
    recording 10mv per degree instead of 1mv.

    Is there a way to do this conversion with a simple op amp? What's the best
    approach to this?

    Thanks
     
  2. Guest

    Your signal ranges from 5mV to 37mV.

    A simple follower-with-gain curcuit will do what you want.

    http://www.physics.brocku.ca/Courses/3P92/lab-manual/392/node16.html

    The National Semiconductor LF353 is probably not the amplifier you'd
    want to use

    http://www.national.com/ds/LF/LF353.pdf

    since the input offset voltage is typically around 5mV with a worst
    case of 13mV over temperature, and it would require positive and
    negative voltage supplies in your application.

    I'd be more inclined to use the LT1006

    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1021,P1211,D1414

    which is a single supply op amp with a 50uV input offest
    specification.

    In principle you could probably use a gain of 100 in your application,
    but you might need to do some heavy low-pass filtering if you did want
    to run with that kind of gain.

    You can build a Sallen and Keyes low pass filter around a single
    follower with gain, but few text-books analyse the circuit operation
    at high gain.

    http://electronicdesign.com/Articles/Index.cfm?ArticleID=7486&pg=2

    The two amplifier version of the circuit - as discussed in Williams
    and Taylor's "Electronic Filter Design Handbook"

    http://www.edacafe.com/books/McGraw_Hill/Electronic_Filter_Design_Handbook/1715-ElecFilter_FM.pdf

    - allows you to choose more or less arbitrary values for the two
    capacitors involved, which makes the design a lot more practicable. E-
    mail me - my e-mail address is real - if you want more detail. The
    dual version of the LT1006 - the LT1013 - would work in such a
    circuit.
     
  3. Winfield

    Winfield Guest

    Hey, Bill, time's ripe for an ASCII schematic!
     
  4. Fred Bloggs

    Fred Bloggs Guest

    There is also this incredibly precision conditioner for which 12-bits is
    a snap:
    http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1009,C1045,P1678
    ....and available for buy off their website.
     
  5. MooseFET

    MooseFET Guest

    On Jan 22, 5:01 pm, wrote:
    [....]
    Add a capacitor across the feedback resistor and you can make the
    filter better and the capacitor values easy to get ones.

    From memory:

    ------------------------------------
    ! !
    === 1000p !
    100K ! 100K !
    ---/\/\----+----/\/\----+-------!+\ !
    ! ! >------+-----+---Out
    0.1 === --!-/ !
    ! ! !
    GND +----/\/\----+ 22K
    ! !
    GND---/\/\---------+-----!!----- 0.1
    220

    This gets quickly to 10 bits, (IIRC) 50mS and reduces the 180Hz by
    (IIRC) 30dB.
     
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