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a simple noise filter, L or C

Discussion in 'General Electronics Discussion' started by DuctDuck, Jun 1, 2013.

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  1. DuctDuck

    DuctDuck

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    Jan 26, 2013
    How could I proceed in selecting a filtering circuit for a DC circuit being measured?

    I have a 5pF capacitor being sensed by a jfet (i.e.-2n5459 gen.purp.); meaning the high impedance gate-source is parallel to the capacitor and its Rdischarge. Right now, 1.5kohm is jfet's Rs and is the input load/Rgen to a DC differential amplifier, meant to amplify the tiny DC fluctuations on the capacitor.

    I don't want to have a parallel capacitor filter the DC fluctations nor do I want a series inductor too either. Possibly 60Hz, or better yet 6Hz, should be eliminated from the DC measurement signal.

    Is there something I'm missing? A compromising point to reconcile with? Please fill me in!
     
  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    A schematic is always helpful.

    Chris
     
  3. DuctDuck

    DuctDuck

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    Jan 26, 2013
    Yes, I quit agree...check out the thumbnail, please. You will see I have went with an RC integrator as the output for a CD jfet amplifier. Keep in mind this is a low DC voltage measuring circuit and not AC. I predict the RC time constant will have to be much smaller than the previous RC time constant.

    p.s.-I would like to deal with the filter issue immediately before anything else.

    p.s.s-I'm getting re-aquainted to using jfets again, so please be patient:)
     

    Attached Files:

  4. duke37

    duke37

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    Jan 9, 2011
    I do not see where the DC signal comes in.

    What frequency do you want to eliminate?
     
  5. DuctDuck

    DuctDuck

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    Jan 26, 2013
    Oh, sorry. The 5pF cap is an air capacitor I built as an ion chamber (i.e.-volume of air and its ions go to the plates), that is where the DC signal comes in.

    The frequencies I want to be rid of are from infernal thermal noise. The S/N calculations point to an extremely low figure; perhaps a low corner frequency Lpf can change it around (e.g.- 60Hz).
     
  6. duke37

    duke37

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    Jan 9, 2011
    I do not see how the ions will go to the plates without a potential to attract them.

    To get a turn over frequency of about 10Hz, you need an R*C time constant of 0.1sec.
    Use C=100nF, R=1M.

    I believe that fets have high low frequency flicker noise.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Biasing a JFET like a BJT is rather odd. It's putting it nearly outside its linear region.

    Chris
     
  8. DuctDuck

    DuctDuck

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    Jan 26, 2013
    Yes, Duke37, yes the potential is there! The design of the capacitor uses a coaxial capacitance calculation, and when 5V is there at either plate with respect to ground or better source, there is enough electrostatics for attracting a minute Coulomb population (i.e.- 23.3625x10-12 A worth as maximum).

    I'm sorry, but I don't want to put any more capacitance on the input stage; after the buffer amp, Cf can then be very small returning minimal energy. No?
     
  9. duke37

    duke37

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    771
    Jan 9, 2011
    Sorry, I just assumed that the voltage supply at the top of the diagram was positive.

    Once you have gone through the fet buffer, you have a low impedance source and a load of 1M will not affect it.
     
  10. DuctDuck

    DuctDuck

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    Jan 26, 2013
    Great, I will go with your low turnover frequency point of 10Hz, et al. Thanks Duke!

    I have noticed some bjt/jfet/mosfet amplifiers using power supply decoupling techniques. Does a CD jfet require this too?
     
  11. duke37

    duke37

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    Jan 9, 2011
    Your + and - supplies need to be very well smoothed as any variation will be fed directly to the 5pF capacitor. A large capacitor on the negative divider would help.
     
  12. DuctDuck

    DuctDuck

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    0
    Jan 26, 2013
    I want to thank the positive efforts of all towards reducing my thermal noise question thread (e.g.- Vnoise=50uVrms, BW=<4kHz).

    I have done some simulations and experiments using your contributions, and some more improvements to analysis and redesigning. This may have castoff your valuable input; sorry Duke, but it was valuable at the beginning of this.

    Because I'm using a bjt differential amplifier I have had to reduce the filter's impedance; at the jfet's source branch. This brought me around to using two approaches to solving my problem; AC signal response and DC transient analysis (sorry I don't know plural for analysis...a-nal-i-se-s but how do you spell it???).

    Rs=4.7kOhm
    Rf=3.3kOhm
    (Rt=<8.2kOhm)

    Cf=10uF
    fc=4.8Hz (and Av=0.01 at f>48Hz...nice)

    Ultimately, the results proved very different from all of your valuable input. Not that you're plain wrong, its the contributions I valued and learned from. So, thanks again and keep on contributing!
     
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