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low capacitance diodes

T

Terry Given

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
Terry said:
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 data/MA3X198 (MA198).pdf

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

John Larkin

Jan 1, 1970
0
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
 
D

Dbowey

Jan 1, 1970
0
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
 
S

Spehro Pefhany

Jan 1, 1970
0
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
 
U

Uwe Bonnes

Jan 1, 1970
0
: 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
 
S

Spehro Pefhany

Jan 1, 1970
0
: 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
 
T

Terry Given

Jan 1, 1970
0
Terry said:
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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