low capacitance diodes

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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



Re: low capacitance diodes

>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


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



Re: low capacitance diodes
>
> 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

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%20data/MA3X198%20(MA198).pdf

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

--
John Popelish


Re: low capacitance diodes
On Tue, 23 Nov 2004 16:34:19 +1300, the renowned Terry Given

>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

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


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: low capacitance diodes

: 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
--
Uwe Bonnes                 snipped-for-privacy@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
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Re: low capacitance diodes
On Tue, 23 Nov 2004 12:31:08 +0000 (UTC), the renowned Uwe Bonnes

>
>: 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

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


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
We've slightly trimmed the long signature. Click to see the full one.
Re: low capacitance diodes
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


Re: low capacitance diodes

>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 :)

Vishay/Siliconix makes some low leakage diodes that may do - the JPAD5
or SSTPAD5 is 0.5 pF, and the JPAD50 is 1.5 pF - Newark has the
SSTPAD5 (smd) in stock, and will sell small quantities.



--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
We've slightly trimmed the long signature. Click to see the full one.
Re: low capacitance diodes

> 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
>

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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