low capacitance diodes

T

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
 
Terry Given wrote:
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
Click to expand...
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
 
On Tue, 23 Nov 2004 16:34:19 +1300, Terry Given <my_name@ieee.org>
wrote:

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
Click to expand...

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
 
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
 
Spehro Pefhany &lt;speffSNIP@interlogdotyou.knowwhat&gt; wrote:

: 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 bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
 
On Tue, 23 Nov 2004 12:31:08 +0000 (UTC), the renowned Uwe Bonnes
&lt;bon@elektron.ikp.physik.tu-darmstadt.de&gt; wrote:

Spehro Pefhany &lt;speffSNIP@interlogdotyou.knowwhat&gt; wrote:

: 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
Click to expand...
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"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
On Tue, 23 Nov 2004 16:34:19 +1300, Terry Given &lt;my_name@ieee.org&gt;
wrote:

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 :)
Click to expand...
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
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
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