low capacitance diodes

Nov 24, 2004 8 Replies
J

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

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J

I needed a similar diode for across a 1 meg resistor. The best I could find was the Panasonic MA3X198.

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

John Popelish
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S

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" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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S

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
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U

: 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

--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

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T

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


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P

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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T

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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