1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

10kHz EM noise

  1. Dec 10, 2006 #1
    Hi all,
    I have a high gain current to voltage amp connected to 2-3 meters of coax which is picking up noise in the 7-10.5 kHz range. Does anyone have any expereince in waht could be the source of this noise? Right now I'm using a couple op-amps and a low pass filter to get the noise to a acceptable level but I would really like to get that bandwidth back. Thanks
  2. jcsd
  3. Dec 10, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Is the amplifier specifically designed to drive the large capacitance of 2-3 meters of coax? It's entirely possible the amplifier is unstable and oscillating in that region of the spectrum.

    - Warren
  4. Dec 10, 2006 #3
    As long as he has his impedances matched the coax should not appear as capacitive. Tell us more please nbo10.
  5. Dec 10, 2006 #4
    It's a two stage amp.
    The first stage has a 100mV/nA gain.
    I can't say much about the first stage beacuse they sanded the part number off the opamp and the circuit is kinda small. But I think it is a AD549(inverting) and 100Mohm feedback resistor. I measured the capactance of the coax I'm using and it's around 500pF. The literature from the Amp manufacturer says that in the worst case I would have 100pF at the input.

    The second stage is pretty basic. There is a Instrumentation Ampilfer that provides a gain of, 1, 10, or 100. Followed by a OP27 with adjustable gain 0.75-1.25. Followed by a simple RC lowpass filter. Then another OP27.

    I put a 10nF cap across the feedback resistor of the first OP27 to help filter the noise. It does an alright job, but I'm worried that I need the bandwidth.

    THe current I'm measuring is about 1nA.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: 10kHz EM noise
  1. Electrical noise (Replies: 10)

  2. PSD of Noise (Replies: 8)

  3. Brown noise (Replies: 1)