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I Op-amps, noise and high frequency/low current input signal

  1. Sep 16, 2016 #1
    Hello everyone,

    In my lab class we've been given task of using op-amps to amplify a current of 1nA and frequency of order of madnitude 1MHz to a discernable level. We've been given breadboards, a selection of different op amps, resistors, an oscilloscope etc to achieve this task. We've made good progress so far as we've gotten down to 10nA. But at 1nA we get to much noise to make a discernible waveform on the oscilloscope. Does any one have any tips on how we may go about reducing noise in this setup?

    Thanks for your time
  2. jcsd
  3. Sep 16, 2016 #2


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    Science Advisor
    Gold Member

    The level of the current is not very relevant as long as the source of that current can drive a relatively large load.

    Since your desired output signal is a voltage, all you need to do is to pass the current through a large resistor and then use the op-amp to amplify the voltage across that resistor;. If you can use a resistor with a value of say 10 MOhm or so you will get a voltage any "normal" op-amp can handle.
    That said, at 1nA you might need to use a few more "tricks". Since you know that the signal is at `1 MHz you can high-pass (or even better band-pass) filter the signal from say 300kHz or so, which will dramatically reduce the amount of noise

    Also, this should probably be in the EE forum.
  4. Sep 16, 2016 #3
    I can't bandpass the signal because the input is meant to be simulating particles hitting a plate at a frequency of say 10000kHz to 1MHz. I've put decoupling capacitors on the opamp so far. I was thinking of putting multiple opamps in stages in series. Would it be better to do it in paralell? What ever little tricks I can do to reduce the noise as much as possible. I usually put the input current through some voltage dividers to reduce the nouse the function generator makes at low ampltidues.
  5. Sep 17, 2016 #4


    Staff: Mentor

    You have misstated the requirement. You do not have an AC signal. You have events arriving at certain rates. If you want to further reduce noise beyond a bandpass, think of an event detector, then count events per second.

    What distinguishes a valid event from noise? If you can't answer that, you can't filter noise.
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