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low capacitance diodes

Discussion in 'Electronic Design' started by Terry Given, Nov 23, 2004.

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  1. Terry Given

    Terry Given Guest

    Hi All,

    I have an application that needs a retrofit - a pair of back-to-back
    "clipping" diodes across an opamp feedback resistor (which cannot be
    reduced below 390k for other reasons)

    I want extremely low capacitance though - 1pF max, 500fF better. a pair
    of 1N4148s drops the gain by 60%; BAS28 and BAV99 each drop the gain by
    38% - better but I'm not there yet (measured not simulated results).

    Any suggestions? Its gotta be smt, and of course cheaper is better :)

    Cheers
    Terry
     
  2. I needed a similar diode for across a 1 meg resistor. The best I
    could find was the Panasonic MA3X198.
    http://rocky.digikey.com/WebLib/Panasonic/Web data/MA3X198 (MA198).pdf

    If you find a better one, I could use it.
     
  3. John Larkin

    John Larkin Guest


    SMS7621-079 (Skyworks, used to be Alpha) parts run about 0.22 pF at 0
    bias, according to my measurements. It's a single in a teeny SC-79
    package, but they have this same chip in a dual SOT-23. It's about the
    lowest capacitance diode you can buy packaged. Skyworks has a dynamite
    free sample kit too... lots of schottkys and varicaps.

    Hey, put two in series and get close to 0.1 pF! It's only rated 2
    volts Vr, but is really better... I have actual reverse leakage curves
    somewhere around here mumble mumble. But a low barrier schottky might
    be a bit leaky across 390K.


    John
     
  4. Dbowey

    Dbowey Guest

    Terry posted:

    <<
    I have an application that needs a retrofit - a pair of back-to-back
    "clipping" diodes across an opamp feedback resistor (which cannot be
    reduced below 390k for other reasons)

    I want extremely low capacitance though - 1pF max, 500fF better. a pair
    of 1N4148s drops the gain by 60%; BAS28 and BAV99 each drop the gain by
    38% - better but I'm not there yet (measured not simulated results).

    Any suggestions? Its gotta be smt, and of course cheaper is better :)
    Did you search on Hot Carrier Diode?

    <A HREF="http://www.alltronics.com/diode_specials.htm">Diode Specials</A> for
    example. You can readily get what you need.

    Don
     
  5. Would a dual PIN diode work? 0.28pF @0V/1GHz for the BAR88-xx series.


    Best regards,
    Spehro Pefhany
     
  6. Uwe Bonnes

    Uwe Bonnes Guest

    : Would a dual PIN diode work? 0.28pF @0V/1GHz for the BAR88-xx series.

    Any source for low numbers of this diode? I don't find it with the usual
    suspects...

    Bye
     
  7. Nope. Last time I needed a few Infineon parts Future had them, but not
    sure about these cell-phone bits.


    Best regards,
    Spehro Pefhany
     
  8. Terry Given

    Terry Given Guest

    Hi All,

    Thanks for your comments. Very helpful. I played with the panasonic
    part, didnt like the pricing of the more esoteric stuff, and went and
    looked harder at the constraints driving the feedback resistor. The
    design equations were yucky when not simplified, but a little work with
    mathcad and I found a way to reduce it, to the point a BAV99 works.
    Who needs closed-form solutions when you have a numerical solver.

    Cheers
    Terry
     
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