Testing a I-V Converter

Discussion in 'The Projects Forum' started by Ando, Nov 2, 2012.

  1. Ando

    Thread Starter New Member

    Nov 2, 2012
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    Hi

    I am working on a current to voltage converter to convert signals in the range of ~300pA into μV, i.e a gain of 10 to the 6.

    I am using an ultra low bias current opamp ( LMP7721) with a 1GΩ resistor. I am having trouble testing this circuit as I cannot easily create a low nA/pA current. I have experimented with voltage dividers etc. but these seem to create impedance issues and my output swings to the negative rail.

    I have also built a voltage to current converter as an input source using a 1MΩ to convert 100mV into 1nA for the input, however I cannot work out how to apply this current to the input of the I-V converter.

    is this a valid test method or am I missing something simpler?

    Best regards

    Sam
     
  2. ramancini8

    Member

    Jul 18, 2012
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    118
    This may be an impossible project for anyone except a very experienced person. 300pA of signal is less than the noise in normal circuits, so you will have problems picking it out of the noise. Also, the noise generated by a 1G ohm resistor is far greater that the signal.

    You need to get on the web and google "low noise electronic designs/amplifiers" to gain an appreciation of your problem. Next google "photo detector circuits" because they have similar design problems. Look at the Hamamatsu Photonics technology seminar book where designs of this type are discussed in detail. Advanced circuit design engineers quiver when handed this problem. Good luck.
     
  3. Ando

    Thread Starter New Member

    Nov 2, 2012
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    Thanks for your reply.
    Are you saying that the testing is near impossible, or the design of the trans impedance amp itself?
    if the circuit is an advanced problem I will abandon it and pursue other avenues, but if it is the testing I will wait until the customer delivers me the sensors which will give me the exact signal I am meant to convert.

    Best regards
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Last edited: Nov 3, 2012
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
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    What is the bandwidth of the signal you are trying to detect? To minimize noise you need to filter the signal so that it is no greater than you need.

    Why not use another 1GΩ resistor as a current source?
     
  6. Ando

    Thread Starter New Member

    Nov 2, 2012
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    Hi

    I have attached my schematic, sorry its only a snapshot of my logbook but I had nothing better to hand.

    I don't need a very high bandwidth at all, I am measuring oxygen cosumption using an amperometric sensor so it should be close to DC maybe? I can't use another 1G resistor as we currently have none to hand and budget on this project is getting quite tight!

    Thanks again

    [​IMG]
     
    Last edited: Nov 8, 2012
  7. Ando

    Thread Starter New Member

    Nov 2, 2012
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  8. Ando

    Thread Starter New Member

    Nov 2, 2012
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    Currently I am just observing -Vdd at the output, I assume this is because nise in the resistor etc. is being amplified?
     
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