Low-C input protection diodes

Low cost diodes, or low overall cost of ownership?

I use PIN diodes; Avago HMPP-3892 is a possibility. It's rated for 1A forward for 1usec max, and comes in a 1.2x1.4mm package.

Questions/comments in your last paragraph are, of course, good ones to address to the manufacturers.

Cheers, Tom

Something like 1N5711 is a classics.

I doubt if there are any discrete diodes with the C < 1pF which are able to survive 1A spike.

It is unlikely to have the problems with ESD because of the typically low impedance and high selectivity of the RF circuits.

What is the circuit? Is this problem due to the ESD or the excessive incoming RF power?

For example, the Linx datasheets often just state abs max at

How about using a tube?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Both. A regular 2-3 cent pair will do just fine, except for being highish in capacitance.

The marketeers spec'd the capacitance of the HMPP at 5V. At 0V it's almost the same as the BAV99 except for a little more package capacitance for the BAV. Figure 2:

Done that already. The answer was pretty much "that's life" ;-)

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Regards, Joerg

http://www.analogconsultants.com/

The capacitance of that one is even higher than the BAV99.

Well, as usual I was hoping there is a free lunch ;-)

They are not at all low-Z. Yeah, for their intended band they may be

50ohms but there is a cap included which is really small. Since it is on the chip it doesn't take much to blow it to pieces.

Mostly excess RF I believe but this is hard to say after the fact.

No kidding, that's exactly what I did for a shortwave pre-amp while in Germany. After skidding down a mossy roof I had it. Built a nice pre-amp around a mil version of the 6AK5W. It never blew and lasted more than a decade until I moved out. Can't do that in this case, plus there are only very few tube mfgs left and the big ones like Sovtek and Svetlana aren't making the old UHF triodes, they concentrate on where the money is, audio folks (BIG bucks) and the transmitter market. People can probably still buy brand new 6L6 tubes when both of us are in a nursing home.

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Regards, Joerg

http://www.analogconsultants.com/

Dual diode, 1 pF typ:

Single diode, 0.5 pF typ, at some unspecified voltage

Quad line protector, 0.6 pF

Multichannel, 1 pF per line:

3 ch, 1.5 pF:

Somebody else, BI or Bourns, has more.

John

Neither dual, or 1 amp but in R.F. use I've never killed a low C, low Vd, BAT81.

Says 1.3pF max at 3.3V but no data for 0V.

This one looks enticing, thanks. No stock at Digikey though but I'll check elsewhere. Normally I shy away when Digikey doesn't have stock but ONSemi is not into vaporware so it should be available (without calling Rebecca...).

Neat! Even has a zener in there for folks who use good old TTL. And 30 Cents for four pairs ain't bad at all, plus Digikey has a whopping

20,000 of them in stock which is a very good sign.

Looks ok but Digikey doesn't carry them and the big yellow end of life message kinda scares me.

Sounds good, too, and under 20 Cents.

Good idea. Somehow I always see Bourns as a resistor company but they do make other stuff.

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Regards, Joerg

http://www.analogconsultants.com/

You mean the BAT18 PIN Diode? That one comes as a dual and is called BAT18-04 for the series pair. But as with a lot of EU parts no stock at Digikey. Also, its capacitance is about the same as that for the old BAV99.

I've killed a lot of RF stuff because sometimes I get involved in power designs. Things like turning the ceramic of a resistor or cap into bubbly green glass. Mostly they look kind of cool after the smoke has wafted off.

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Regards, Joerg

http://www.analogconsultants.com/

0.46pF isn't significantly less than 0.62pF? And at 1GHz, it's about . 32pF; don't know that you'll get that benefit with the BAV99. At a couple volts reverse, the ratio's even better. I didn't bother checking the BAV99 before posting, since you'd said it was 3-4pF for two. I assumed you wanted something somewhat better than 3-4pF for a pair. Guess I'll know better than to trust things like that in the future.

Cheers, Tom

Thanks for that wide-ranging list John. Which of these parts do you use regularly?

Sorry, that was with the pad and all that. So if one comes in a better housing than the SOT23 of the BAV99 that would be good but the diodes themselves don't seem to be too different.

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Regards, Joerg

http://www.analogconsultants.com/

Indeed a good list. If Digikey qties on hand are an indicator the ST part seems to be in favor with voters these days.

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Regards, Joerg

http://www.analogconsultants.com/

Le Tue, 23 Oct 2007 14:40:09 +0000, Joerg a écrit:

BAV99T in SC75/SOT523 package, from infineon (yeah, I know), Diode INc, and MMC.

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Thanks,
Fred.

So what you're saying is that, with 1.3pF in the diode pair (at ZERO bias), you have 2pF in the single pad, 0.8x0.9mm? So the ground plane is less than 0.02mm (less than a mil) under that pad? You obviously don't have a diode problem, you have a layout problem. You're right, cutting the diode capacity to ZERO isn't all that much help, and it's also not very realistic. On the other hand, it's also obvious you didn't look very closely at the package for the part I suggested; on the same scale (same ground plane less than a mil away), its pads would be around 0.3pF each. Even with your (apparently) ridiculously thin dielectric, its pads can be integrated into a 50 ohm line with very little effect. Even with a SOT23, though, protecting a microstrip on FR4 with 20 mil dielectric should not introduce any discontinuity; the pad "capacitance" (well under 2pF anyway) is just integrated into the line impedance. An 0.8mm trace width (the width of a SOT23 pad) 0.5mm (20 mils) above a plane at Er=4.7 gives you about a 50 ohm line, at 0.12pF/mm. If the SOT23 straddles the line, you can neck the line down a bit where it passes between the pins 1 and 3 to maintain 50 ohms. Then it becomes an issue only of diode capacitance, and it does seem to me that getting rid of nearly half the capacitance is worthwhile.

Even if it WERE 3pF across 50 ohms, that's only a quarter dB or so at

500MHz. With the suggested diode-pair, at zero bias, you can get that down to well under 0.1dB, and unless the design is remarkably clean or simple, the rest of it won't be nearly that good.

Cheers, Tom

On Oct 22, 2:31 pm, Vladimir Vassilevsky wrote: ...

Data sheet rating of my previously-recommended HMPP-389x diodes is 1 amp max for 1usec; capacitance at zero bias is typically 0.46pF at

1MHz, 0.32pF at 1GHz, typical per diode. I suppose other PIN diodes would provide similar performance.

Cheers, Tom

BTW, it isn't yet known what is the cause of the fault: is it ESD or the excess RF power at the input or something else. Depending on that, the protection measures should be quite different.

There is a great deal of small size low-C protection devices intended for the fast USB and FireWire.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

None, actually.

We do use a lot of Skyworks low-c schottky diodes, 0.2 pF range, but not for esd protection.

John

That number came from datasheets and app notes and includes the SOT23 package as well. But as I said if the diode has a better package it would lower the total. The HMPP in the MiniPak you suggested would certainly have a better package. However, the other downside is that it's around $0.40. A bit steep in this application versus the usual $0.02 for a dual.

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Regards, Joerg

http://www.analogconsultants.com/

I try to stay away from Infineon unless Digikey has stock. Got burned one too many times. Should ideally be from ON Semi or another major US or Asian mfg, or ST. The BAV199 is also pretty cool, also SC75 and sometimes half a penny less in quantities.

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Regards, Joerg

http://www.analogconsultants.com/