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How to measure noise from a current source

  1. Jun 22, 2012 #1
    I'm asked to get the noise signal from the constant current source I built with a 741 Op-Amp. I know I have to read it from the scope, but how do I go about the connections to the scope, and what other procedures must I follow to read the noise contribution? Pls refer to the figure provided in the link below to see the circuit.

    http://www.freeimagehosting.net/n92gg
     
  2. jcsd
  3. Jun 22, 2012 #2
    I looked at the circuit quickly, I don't quite understand the circuit nor would I do it this way to get a constant current source. If you use resistors to simulate a current source, the output impedance is way too low unless you use very high value resistors which would limit you to very low current capability. Also, it only work if the current injection point is ground referenced. If the point is moving, all bets are off. It is much easier to make a current source with an op-amp and a MOSFET.

    But be that as it may. To measure the noise, you terminate the output with a resistor and measure the noise. You can use a RF noise meter for measurement. It is possible to measure with the scope, but it is not accurate because it is hard to determine the exact amplitude. I did a lot of noise measurement with scope just because I did not have the noise meter. I had to make judgement what is the amplitude as all you see is a fuzzy line on the screen!!!

    The difficult part is you likely have to put amplifier stages to amplify the signal before you can use the noise meter and the the amplifier themselves produce noise. With the noise meter, it's easier, you put a resistor at the input of the amplifier chain, measure the noise. Then connect the current source and see the increase of the noise. Then you do calculation and get the noise contribution of the current source alone. Remember, noise are root mean square, not direct addition.
     
  4. Jun 25, 2012 #3
    Additional Details
    I am sorry if I am speaking unprofessionally. How do I get to know the frequency range of the measured noise? I made some amendments to the circuit, and have attached it in the link below, together with the output I got from the scope. How do I know the noise frequency range? I hope the scope output I attached is correct, because I am a novice as per the use of the Agilent scope. Thanks.

    http://imageshack.us/photo/my-images/585/justrw.jpg/
     
  5. Jun 25, 2012 #4
    Current noise is wide band to very high frequency. We mostly talk about to the frequency limited to the frequency of interest.

    The frequency of the current noise of your circuit is the limit of the frequency response of your circuit. Say, it your amp has a -3dB frequency at 1MHz, then you only need to worry about noise slightly higher than 1MHz and that's it. Say 2MHz or even less.

    I cannot read you graph because some PC stuff is running and checking my computer when I open the link and I don't like it!!!!
     
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