opamp offset

Discussion in 'General Electronics Chat' started by MikeA, Jan 11, 2014.

  1. MikeA

    Thread Starter Member

    Jan 20, 2013
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    I'm trying to amplify a signal in the mV range. Say I have a base signal of 1.000v. And I want to amplify that 3000 times, so 1.001v signal will output as 3v from opamp output.

    So I'll have a reference voltage of 1.000v fed into the opamp, and have it listen to the signal. Am I correct in that if I want to measure 1mv accurately, I need an opamp with an input offset voltage spec much lower than 1mv for it to work correctly?
     
  2. MikeA

    Thread Starter Member

    Jan 20, 2013
    125
    17
    p.s. why can't a post subject line be longer than like 12 characters?:confused: descriptive longer subject doesn't post
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Your descriptions has something wrong.
    The base voltage is 1.000V.
    then 1.000v * 3000 = 3KV, the output is 3KV?

    Or your base voltage is 1mV, so the output equal to 1mV * 3000 = 3V.

    It depends on how low the input offset voltage you want, you can see the OP07,OP27,OP37 or LF356,LF357.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    You can use the reference supply to get rid of the opamp's offset (make it adjustable to null out the offset). The problem is the temperature dependence of the offset...
     
  5. MikeA

    Thread Starter Member

    Jan 20, 2013
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    MikeML, do you mean I'll have trouble with the offset voltage drifting with temperature? Is there a better approach to what I'm trying to do then?
     
  6. ramancini8

    Member

    Jul 18, 2012
    442
    118
    1. Pick a very low offset op amp; they usually have excellent drift parameters. Don't forget the resistors because the cheap 5% type usually have poor tempco.
    2. If the signal is ac use a blocking capacitor.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Are you primarily concerned with the ∆V between the two signals, with both voltages possibly drifting around, or do you want a ∆V against an absolute reference that is as solid as you can get it? A differential op-amp with a gain of 3000 will give you the former. Both voltages need to be within the common mode input range and ∆V*gain must remain within the output range of the op-amp.
     
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