TAPR TICC boxed
The FJH1100 is specified for reverse leakage of 10pA at 15v (which is
also the absolute maximum working voltage), and 3pA reverse leakage at
5v. Junction capacitance is 2pF. They cost $8.90 each at Mouser.
The B-C junction of an MPSH10 or MMBTH10 (SMT version) has only half as
much reverse leakage current (5pA) at a higher reverse voltage (20v). I
just measured a few MPSH10s at 5v, and they showed less than 1pA reverse
leakage. The maximum working voltage is 30v and junction capacitance is
0.7pF. Switching times are 5-10x faster than the FJH1100. MMBTH10s
cost $0.22 each at Mouser. MMBT5179s (SMT version of 2N5179) are very
similar and cost $0.26 each at Mouser.
I have used the B-C junctions of BJTs and the gate junctions of JFETS as
low-leakage diodes for many, many years, for exactly these reasons
(better performance than "ultra low leakage" signal diodes and much
lower cost).
Best regards,
Charles
On 3/31/2017 9:39 PM, Alex Pummer wrote:
FJH1100
Ultra Low Leakage Diode
Alex
On 3/31/2017 6:00 PM, Charles Steinmetz wrote:
Mark wrote:
I thought about using the clamp diodes as protection but was a bit
worried about power supply noise leaking through the diodes and
adding some jitter to the input signals...
It is a definite worry even with a low-noise, 50 ohm input, and a
potential disaster with a 1Mohm input. Common signal diodes (1N4148,
1N914, 1N916, 1N4448, etc.) are specified for 5-10nA of reverse
current. Even a low-leakage signal diode (e.g., 1N3595) typically has
several hundred pA of leakage. Note that the concern isn't just power
supply noise -- the leakage current itself is quite noisy.
For low-picoamp diodes at a decent price, I use either (1) the B-C
diode of a small-signal BJT, or (2) the gate diode of a small-geometry
JFET. A 2N5550 makes a good high-voltage, low-leakage diode with
leakage current of ~30pA. Small signal HF transistors like the MPSH10
and 2N5179 (and their SMD and PN variants) are good for ~5pA, while
the gate diode of a PN4417A JFET (or SMD variant) has reverse leakage
current of ~1pA (achieving this in practice requires a very clean
board and good layout).
I posted some actual leakage test results to Didier's site, which can
be downloaded at
http://www.ko4bb.com/getsimple/index.php?id=download&file=03_App_Notes_-_Proceedings/Reverse_leakage_of_diode-connected_BJTs_and_FETs_measurement_results.pdf.
This document shows the connections I used to obtain the data.
The TICC doesn't have the resolution for it to matter or justify a
HP5370 or better quality front end. I'll probably go with a fast
comparator to implement the variable threshold input.
Properly applied, a fast comparator will have lower jitter than the
rest of the errors, and is an excellent choice. Bruce suggested the
LTC6752, which is a great part if you need high toggle speeds (100s of
MHz) or ultra-fast edges. But you don't need high toggle rates and
may not need ultra-fast edges. Repeatability and stability are more
important than raw speed in this application. The LT1719, LT1720, or
TLV3501 may work just as well for your purpose, and they are
significantly less fussy to apply.
Note that the LTC6752 series is an improved replacement for the
ADCMP60x series, which itself is an improved replacement for the
MAX999. Of these three, the LTC6752 is the clear winner in my tests.
If you do choose it (or similar), make sure you look at the
transitions with something that will honestly show you any chatter at
frequencies up to at least several GHz. It only takes a little
transition chatter to knock the potential timing resolution of the
ultra-fast comparator way down. Do make sure to test it with the
slowest input edges you need it to handle.
Best regards,
Charles
Hi
One interesting “feature” of leakage specs:
They often reflect the measurement limit rather than the actual device performance. If they
are guaranteed by test, the limit may be orders of magnitude above the actual performance.
That’s on top of the likely “rated at max temperature” part that is relatively easy to understand.
(A measurement at 125C will show a lot more leakage than one at 25 C).
Often measuring a representative sample under reasonable conditions is the only way to come
up with useful information.
Bob
On Apr 2, 2017, at 5:12 PM, Charles Steinmetz csteinmetz@yandex.com wrote:
The FJH1100 is specified for reverse leakage of 10pA at 15v (which is also the absolute maximum working voltage), and 3pA reverse leakage at 5v. Junction capacitance is 2pF. They cost $8.90 each at Mouser.
The B-C junction of an MPSH10 or MMBTH10 (SMT version) has only half as much reverse leakage current (5pA) at a higher reverse voltage (20v). I just measured a few MPSH10s at 5v, and they showed less than 1pA reverse leakage. The maximum working voltage is 30v and junction capacitance is 0.7pF. Switching times are 5-10x faster than the FJH1100. MMBTH10s cost $0.22 each at Mouser. MMBT5179s (SMT version of 2N5179) are very similar and cost $0.26 each at Mouser.
I have used the B-C junctions of BJTs and the gate junctions of JFETS as low-leakage diodes for many, many years, for exactly these reasons (better performance than "ultra low leakage" signal diodes and much lower cost).
Best regards,
Charles
On 3/31/2017 9:39 PM, Alex Pummer wrote:
FJH1100
Ultra Low Leakage Diode
Alex
On 3/31/2017 6:00 PM, Charles Steinmetz wrote:
Mark wrote:
I thought about using the clamp diodes as protection but was a bit
worried about power supply noise leaking through the diodes and
adding some jitter to the input signals...
It is a definite worry even with a low-noise, 50 ohm input, and a
potential disaster with a 1Mohm input. Common signal diodes (1N4148,
1N914, 1N916, 1N4448, etc.) are specified for 5-10nA of reverse
current. Even a low-leakage signal diode (e.g., 1N3595) typically has
several hundred pA of leakage. Note that the concern isn't just power
supply noise -- the leakage current itself is quite noisy.
For low-picoamp diodes at a decent price, I use either (1) the B-C
diode of a small-signal BJT, or (2) the gate diode of a small-geometry
JFET. A 2N5550 makes a good high-voltage, low-leakage diode with
leakage current of ~30pA. Small signal HF transistors like the MPSH10
and 2N5179 (and their SMD and PN variants) are good for ~5pA, while
the gate diode of a PN4417A JFET (or SMD variant) has reverse leakage
current of ~1pA (achieving this in practice requires a very clean
board and good layout).
I posted some actual leakage test results to Didier's site, which can
be downloaded at
http://www.ko4bb.com/getsimple/index.php?id=download&file=03_App_Notes_-_Proceedings/Reverse_leakage_of_diode-connected_BJTs_and_FETs_measurement_results.pdf.
This document shows the connections I used to obtain the data.
The TICC doesn't have the resolution for it to matter or justify a
HP5370 or better quality front end. I'll probably go with a fast
comparator to implement the variable threshold input.
Properly applied, a fast comparator will have lower jitter than the
rest of the errors, and is an excellent choice. Bruce suggested the
LTC6752, which is a great part if you need high toggle speeds (100s of
MHz) or ultra-fast edges. But you don't need high toggle rates and
may not need ultra-fast edges. Repeatability and stability are more
important than raw speed in this application. The LT1719, LT1720, or
TLV3501 may work just as well for your purpose, and they are
significantly less fussy to apply.
Note that the LTC6752 series is an improved replacement for the
ADCMP60x series, which itself is an improved replacement for the
MAX999. Of these three, the LTC6752 is the clear winner in my tests.
If you do choose it (or similar), make sure you look at the
transitions with something that will honestly show you any chatter at
frequencies up to at least several GHz. It only takes a little
transition chatter to knock the potential timing resolution of the
ultra-fast comparator way down. Do make sure to test it with the
slowest input edges you need it to handle.
Best regards,
Charles
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Low current measurements take a lot of time on the automatic test
equipment and time in this case is measured in seconds. The same
applies to low frequency noise.
For an example, take a look at the National (now TI) LMC6001 and
LMC6081:
Unlike National, TI does not care about input bias current in their
selection guides so you will have to look that up in the datasheets:
http://www.ti.com/product/lmc6001
http://www.ti.com/product/lmc6081
The difference in the parts is that the LMC6001 is tested for an Ib of
25fA and below and this is reflected in the price which is $5.76
instead of the $0.83 of the LMC6081.
Right about the time that the LMC6001 was released, Robert Pease wrote
some articles talking about the bias current testing and the
economics.
The same thing applies to all of those small signal transistors with
25, 50, and 100nA leakage specifications. Those numbers are simply
good enough for typical applications and what the tester can handle in
the time allotted and have nothing to do with the actual transistor
performance.
So collector-base junctions make good low leakage high voltage diodes
although they are slow which does not normally matter for an input
protection circuit and may even be preferable. Emitter-base junctions
make good low leakage fast diodes but with low breakdown voltages.
The cheapest guaranteed low leakage diode is probably some variety of
4117/4118/4119 n-channel JFET.
A protection diode needs to also have a fast turn on with little or no overshoot of the forward voltage.
Reverse recovery time can be an issue if one is relying on the clamp for protection against a periodic overload such as when an input is overdriven by a sinewave input and one wishes to make useful measurements whilst this occurs.
The internal protection diodes of HCMOS devices can severely degrade the device propagation delay jitter when they conduct.
Bruce
On 05 April 2017 at 06:05 David davidwhess@gmail.com wrote:
Low current measurements take a lot of time on the automatic test
equipment and time in this case is measured in seconds. The same
applies to low frequency noise.
For an example, take a look at the National (now TI) LMC6001 and
LMC6081:
Unlike National, TI does not care about input bias current in their
selection guides so you will have to look that up in the datasheets:
http://www.ti.com/product/lmc6001
http://www.ti.com/product/lmc6081
The difference in the parts is that the LMC6001 is tested for an Ib of
25fA and below and this is reflected in the price which is $5.76
instead of the $0.83 of the LMC6081.
Right about the time that the LMC6001 was released, Robert Pease wrote
some articles talking about the bias current testing and the
economics.
The same thing applies to all of those small signal transistors with
25, 50, and 100nA leakage specifications. Those numbers are simply
good enough for typical applications and what the tester can handle in
the time allotted and have nothing to do with the actual transistor
performance.
So collector-base junctions make good low leakage high voltage diodes
although they are slow which does not normally matter for an input
protection circuit and may even be preferable. Emitter-base junctions
make good low leakage fast diodes but with low breakdown voltages.
The cheapest guaranteed low leakage diode is probably some variety of
4117/4118/4119 n-channel JFET.
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and follow the instructions there.
On Wed, 5 Apr 2017 09:13:34 +1200 (NZST), you wrote:
A protection diode needs to also have a fast turn on with little or no overshoot of the forward voltage.
That would be ideal but forward turn on time is rarely specified and
usually assumed to be fast and some fast diodes have appallingly slow
turn on. This is one of those things that needs to be qualified or
selected for if it is important.
I suspect there is some obscure processing issue with diodes that
causes slow turn-on that does not show up in transistors.
Reverse recovery time can be an issue if one is relying on the clamp for protection against a periodic overload such as when an input is overdriven by a sinewave input and one wishes to make useful measurements whilst this occurs.
Definitely.
The internal protection diodes of HCMOS devices can severely degrade the device propagation delay jitter when they conduct.
Bruce
They sure can but isn't this because of minority carrier injection? I
wonder if this is only a problem with junction isolated integrated
circuit processes. I probably knew at one point but forgot.
Dual and quad analog ICs can suffer from a different problem where
exceeding the common mode input voltage range screws up common bias
circuits causing other elements to malfunction.
David wrote:
So collector-base junctions make good low leakage high voltage diodes
although they are slow
I guess it depends on what one means by "slow" and "fast."
The B-C junction of an MPSH10/MMBTH10 or 2N/PN/MMBT5179 switches on in
<1nS and off in <2nS, which is comparable with Schottky microwave mixer
diodes such as the Agilent HSMS282x series and better than "ultra-fast"
silicon switching diodes such as the FD700 and 1S1585. (I did my
switching tests at 20mA.) (Note that the silicon and Schottky switching
diodes have reverse leakage currents from several hundred to tens of
thousands of times higher than the B-C junction of an MPSH10/MMBTH10.)
The gate junction of a 2N/PN/MMBF4117A JFET switches on in <2nS and off
in <4nS.
The cheapest guaranteed low leakage diode is probably some variety of
4117/4118/4119 n-channel JFET.
If the 5pA reverse leakage current of the MPSH10/MMBTH10 is too much and
one must, must, must get leakage down to 1pA, the 2N/PN/MMBF4117A is the
best inexpensive choice that I'm aware of.
Best regards,
Charles
On Wed, 5 Apr 2017 02:40:13 -0400, you wrote:
David wrote:
So collector-base junctions make good low leakage high voltage diodes
although they are slow
I guess it depends on what one means by "slow" and "fast."
I was referring to within the same transistor; emitter-base junctions
are much faster than collector-base junctions.
The B-C junction of an MPSH10/MMBTH10 or 2N/PN/MMBT5179 switches on in
<1nS and off in <2nS, which is comparable with Schottky microwave mixer
diodes such as the Agilent HSMS282x series and better than "ultra-fast"
silicon switching diodes such as the FD700 and 1S1585. (I did my
switching tests at 20mA.) (Note that the silicon and Schottky switching
diodes have reverse leakage currents from several hundred to tens of
thousands of times higher than the B-C junction of an MPSH10/MMBTH10.)
I have never actually tried this with RF transistors. I know one
thing to watch out for if you are looking for low leakage is gold
doping and some less that reputable manufacturers "cheat" in this
respect so transistors used as low leakage diodes should be at least
qualified by manufacturer which is a problem with counterfeits and
unscrupulous purchasing managers.
Out of curiosity, and I tried to look this up years ago, what doping
is used for PNP RF transistors and saturated switches if it is not
gold? Does it also increase leakage?
And I have another question if you know. How is rb'Cc measured?
Tektronix at some point was grading 2N3906s for rb'Cc < 50ps.
The gate junction of a 2N/PN/MMBF4117A JFET switches on in <2nS and off
in <4nS.
The cheapest guaranteed low leakage diode is probably some variety of
4117/4118/4119 n-channel JFET.
If the 5pA reverse leakage current of the MPSH10/MMBTH10 is too much and
one must, must, must get leakage down to 1pA, the 2N/PN/MMBF4117A is the
best inexpensive choice that I'm aware of.
The advantage of the 4117/4118/4119 is that the leakage is already
tested to a given specification so no qualification or testing is
necessary.
David wrote:
I know one thing to watch out for if you are looking for low
leakage is gold doping
Anything that increases carrier mobility increases leakage current (all
else equal -- i.e., for each particular device geometry). This accounts
for the much higher leakage of Schottky and germanium junctions.
Out of curiosity, and I tried to look this up years ago, what doping
is used for PNP RF transistors and saturated switches if it is not
gold? Does it also increase leakage?
Gold doping doesn't affect the speed of BJTs in the active region very
much -- its purpose is to reduce minority carrier lifetime and, thereby,
to reduce storage time when a transistor recovers from saturation. I'm
not sure how manufacturers deal with this in the case of PNPs. [Note
that the list of fast PNP small-signal switching transistors is very
short, and the fastest of them are slower than the slowest fast NPN
switches.]
And I have another question if you know. How is rb'Cc measured?
One way is to drive the transistor with a medium-high frequency (well
down the 1/f portion of its current gain curve -- typically 10-50MHz for
small-signal BJTs) and measure the base-collector phase shift. It can
also be calculated from fT and Cc-b. There is a JEDEC standard for
measuring rb'Cc, but I'm not finding my copy at the moment. It may be
posted on the JEDEC web site.
The advantage of the 4117/4118/4119 is that the leakage is already
tested to a given specification so no qualification or testing is
necessary.
That may be true, but there is nothing in the data published by Vishay,
Fairchild, Calogic, or InterFET to indicate this. Spot-checking, along
with the part design, should be sufficient to guarantee meeting the
spec. I'll try to remember to ask the Vishay process engineer next time
I talk to her.
Best regards,
Charles
Another thing to watch out for if you need very low leakage, is if the
package is transparent. All junctions are photodiodes.
Maybe it's less of a problem now with SMTs, than it was with glass body
diodes or translucent transistor packages.
Andy
David wrote:
what doping is used for PNP RF transistors and saturated switches
if it is not gold? Does it also increase leakage?
I replied:
Gold doping doesn't affect the speed of BJTs in the active region very
much -- its purpose is to reduce minority carrier lifetime and, thereby,
to reduce storage time when a transistor recovers from saturation. I'm
not sure how manufacturers deal with this in the case of PNPs.
After I posted, I recalled learning in a long-ago device physics course
that both Gold and Platinum doping were used to reduce minority carrier
lifetime in PNP saturated switches. According to Motorola, the
MPS3639/3640, 2N4209, and 2N5771 were gold-doped PNP saturated switches
(all are now obsolete, although SMD versions of the 3640 and 5771 appear
to still be available).
And yes, doping PNPs with either Gold or Platinum does increase reverse
leakage current (Platinum less so than Gold).
Best regards,
Charles