Low-C input protection diodes

[Joerg]

Hello Folks,

What's the low cost and most of all low capacitance diode pair du jour
for protecting RF chip inputs? My staple is the BAV99 or BAV199 but
above 500MHz the 3-4pF of a pair begin to matter. Can't bias them much
because there ain't a lot of VCC to begin with.

Recovery time doesn't matter as long as the stray inductance is low so
the diode kicks in the nanosecond a spike wants to come in. They should
be able to stomach something around an amp once in a blue moon. Series
connection, of course. If it has to be singles, ok.

I was pretty surprised to learn that some RF chips have internal
coupling caps but almost zilch in terms of protection. Consequently a
client of mine has them die like flies. Why are RF chip inputs so
fickle? For example, the Linx datasheets often just state abs max at
0dBm. Great. That's not much of a consolation for anyone who has to
deploy in a rough environment.

 

[Tom Bruhns]

Hello Folks,

What's the low cost and most of all low capacitance diode pair du jour
for protecting RF chip inputs? My staple is the BAV99 or BAV199 but
above 500MHz the 3-4pF of a pair begin to matter. Can't bias them much
because there ain't a lot of VCC to begin with.

Recovery time doesn't matter as long as the stray inductance is low so
the diode kicks in the nanosecond a spike wants to come in. They should
be able to stomach something around an amp once in a blue moon. Series
connection, of course. If it has to be singles, ok.

I was pretty surprised to learn that some RF chips have internal
coupling caps but almost zilch in terms of protection. Consequently a
client of mine has them die like flies. Why are RF chip inputs so
fickle? For example, the Linx datasheets often just state abs max at
0dBm. Great. That's not much of a consolation for anyone who has to
deploy in a rough environment.

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

 

[Vladimir Vassilevsky]

Hello Folks,

What's the low cost and most of all low capacitance diode pair du jour
for protecting RF chip inputs?

Something like 1N5711 is a classics.

My staple is the BAV99 or BAV199 but
above 500MHz the 3-4pF of a pair begin to matter. Can't bias them much
because there ain't a lot of VCC to begin with.
Recovery time doesn't matter as long as the stray inductance is low so
the diode kicks in the nanosecond a spike wants to come in. They should
be able to stomach something around an amp once in a blue moon. Series
connection, of course. If it has to be singles, ok.

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

I was pretty surprised to learn that some RF chips have internal
coupling caps but almost zilch in terms of protection.

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

Consequently a
client of mine has them die like flies. Why are RF chip inputs so
fickle?

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

0dBm. Great. That's not much of a consolation for anyone who has to
deploy in a rough environment.

How about using a tube?


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Joerg]

Low cost diodes, or low overall cost of ownership?

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

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.

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:

http://www.avagotech.com/assets/downloadDocument.do?id=262

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

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

 

[Joerg]

Something like 1N5711 is a classics.

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

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

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

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

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.

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

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

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

How about using a tube?

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.

 

[John Larkin]

Hello Folks,

What's the low cost and most of all low capacitance diode pair du jour
for protecting RF chip inputs? My staple is the BAV99 or BAV199 but
above 500MHz the 3-4pF of a pair begin to matter. Can't bias them much
because there ain't a lot of VCC to begin with.

Recovery time doesn't matter as long as the stray inductance is low so
the diode kicks in the nanosecond a spike wants to come in. They should
be able to stomach something around an amp once in a blue moon. Series
connection, of course. If it has to be singles, ok.

I was pretty surprised to learn that some RF chips have internal
coupling caps but almost zilch in terms of protection. Consequently a
client of mine has them die like flies. Why are RF chip inputs so
fickle? For example, the Linx datasheets often just state abs max at
0dBm. Great. That's not much of a consolation for anyone who has to
deploy in a rough environment.

Dual diode, 1 pF typ:

http://www.semiconwell.com/jd_net/swdl001.htm


Single diode, 0.5 pF typ, at some unspecified voltage

http://www.onsemi.com/PowerSolutions/product.do?id=ESD9L


Quad line protector, 0.6 pF

http://www.st.com/stonline/products/literature/ds/11599/dviulc6-4sc6.htm



Multichannel, 1 pF per line:

http://www.cmd.com/products/dds.php?product=cm1213


3 ch, 1.5 pF:

http://focus.ti.com/docs/prod/folders/print/tpd3e001.html


Somebody else, BI or Bourns, has more.

John

 

[john jardine]

Hello Folks,

What's the low cost and most of all low capacitance diode pair du jour
for protecting RF chip inputs? My staple is the BAV99 or BAV199 but
above 500MHz the 3-4pF of a pair begin to matter. Can't bias them much
because there ain't a lot of VCC to begin with.

Recovery time doesn't matter as long as the stray inductance is low so
the diode kicks in the nanosecond a spike wants to come in. They should
be able to stomach something around an amp once in a blue moon. Series
connection, of course. If it has to be singles, ok.

I was pretty surprised to learn that some RF chips have internal
coupling caps but almost zilch in terms of protection. Consequently a
client of mine has them die like flies. Why are RF chip inputs so
fickle? For example, the Linx datasheets often just state abs max at
0dBm. Great. That's not much of a consolation for anyone who has to
deploy in a rough environment.

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

 

[Joerg]

Dual diode, 1 pF typ:

http://www.semiconwell.com/jd_net/swdl001.htm

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

Single diode, 0.5 pF typ, at some unspecified voltage

http://www.onsemi.com/PowerSolutions/product.do?id=ESD9L

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...).

Quad line protector, 0.6 pF

http://www.st.com/stonline/products/literature/ds/11599/dviulc6-4sc6.htm

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.

Multichannel, 1 pF per line:

http://www.cmd.com/products/dds.php?product=cm1213

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

3 ch, 1.5 pF:

http://focus.ti.com/docs/prod/folders/print/tpd3e001.html

Sounds good, too, and under 20 Cents.

Somebody else, BI or Bourns, has more.

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

 

[Joerg]

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

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.

 

[Tim Williams]

Can't do that in this case, plus there are only very few tube mfgs left

What's that matter? The surplus supply is still well stocked. No need to
use exclusively JAN-CHS 6AK5-ABCXYZ's either. 6AU6, 6BA6, 6DK6, 6DJ8
(well, not a pentode, but often used in RF anyways), 12AT7, 6BQ7, the list
goes on for ages. And equivalents, and similar or equivalent European and
Russian types.

Assuming you were going to use a tube, what is it about
not-current-production types that matters so much? Are you making ten
thousand of these things? Now I want to know...

Tim

 

[Tom Bruhns]

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:

http://www.avagotech.com/assets/downloadDocument.do?id=262


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

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

 

[Winfield]

Dual diode, 1 pF typ:
http://www.semiconwell.com/jd_net/swdl001.htm

Single diode, 0.5 pF typ, at some unspecified voltage
http://www.onsemi.com/PowerSolutions/product.do?id=ESD9L

Quad line protector, 0.6 pF
http://www.st.com/stonline/products/literature/ds/11599/dviulc6-4sc6.htm

Multichannel, 1 pF per line:
http://www.cmd.com/products/dds.php?product=cm1213

3 ch, 1.5 pF:
http://focus.ti.com/docs/prod/folders/print/tpd3e001.html

Somebody else, BI or Bourns, has more.

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

 

[Joerg]

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.

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.

 

[Joerg]

What's that matter? The surplus supply is still well stocked. No need to
use exclusively JAN-CHS 6AK5-ABCXYZ's either. 6AU6, 6BA6, 6DK6, 6DJ8
(well, not a pentode, but often used in RF anyways), 12AT7, 6BQ7, the list
goes on for ages. And equivalents, and similar or equivalent European and
Russian types.

Assuming you were going to use a tube, what is it about
not-current-production types that matters so much? Are you making ten
thousand of these things? Now I want to know...

Yes, per year. But I am not at liberty to say what it is ;-)

 

[Joerg]

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

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

 

[Fred Bartoli]

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

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.

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

 

[Tom Bruhns]

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.

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

 

[Tom Bruhns]

On Oct 22, 2:31 pm, Vladimir Vassilevsky <[email protected]>
wrote:
....

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

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

 

[Vladimir Vassilevsky]

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.

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
http://www.abvolt.com

 

[John Larkin]

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

None, actually.

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

John