Current to voltage converter

Discussion in 'General Electronics Chat' started by legolas11, Dec 10, 2014.

  1. legolas11

    Thread Starter Member

    Feb 26, 2013
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    i would like to measure very small current (.1 uA - 20 uA) and the output voltage limit is 6 volt. is the attached circuit ok for this??

    Thanks
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    The 358 has quite a bit of input bias offset so you might want an instrumentation amplifier.
     
  3. #12

    Expert

    Nov 30, 2010
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    To keep the error down to 1% you need an input current of 1 na or less. www.mouser.com shows 2018 op-amps that can do that. (Use the search engine on a vendors site.)
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What does the ground symbol connect to? That is importaint for the common mode voltage.

    What TWO points do you use to measure U1A-1 (the op amp output)?
     
  5. legolas11

    Thread Starter Member

    Feb 26, 2013
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    i have INA121 and it says bias current as low as 4 pA. if i want to use it for the purpose, do i need any other modification???
     
  6. legolas11

    Thread Starter Member

    Feb 26, 2013
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    i wanted to use woth available one....currently i have ina 121, AD823, lm 393....seems like ina121 would be suitable...
     
  7. legolas11

    Thread Starter Member

    Feb 26, 2013
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    i did not test it yet. i think i would connect it to the negative terminal of source (current source) . is there any problem to do that??

    i would like to directly read the output through data logger. so i am thinking of connecting the output terminal of U1-A (pin 1) and ground point with data logger +/- connections.
     
  8. legolas11

    Thread Starter Member

    Feb 26, 2013
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    I am trying to test the circuit by supplying current (1.5 mA) using the CC mode of a DC supply. But i can not identify any change in ouput voltage with the change in feedback resistance(200 ohm - 1 kohm). I am using 12 volt single power supply. and the output i am recieveing around 1.3 volt. i attached a multimeter in series with input and the current remain almost constant with the change of resitors. can anyone tell me what is excatly wrong here. I tried using 3 different ICs lm358,lm393 and ltc1052. but i could not amplify the signal with either of them.
     
  9. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    You may want to study Mr David Jones' design against yours. He gives a very clear explanation for each one of the components. He's also got a YouTube channel that you may want to check out. See attached file.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    The INA121 is an instrumentation amplifier and is not suitable. They have internal feedback circuits and normally are not used with any external feedback.
    You need an op amp for your circuit. The AD823 should work fine with 800μV maximum input offset voltage and 25pA maximum input bias current.
     
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