Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

They are relevant when a high input impedance buffer is used making it
easier to add series/shunt overload protection.  Protecting against
400 volts and higher is feasible this way.

Adding overload protection to a 50 ohm input is an interesting
challenge but it can be done.  Precede the 50 ohm termination if used
with a current driven diode bridge.  The current then determines the
maximum input voltage which can be passed to the 50 ohm termination
and following circuits.  Check out the design of the Tektronix 7A29
vertical amplifier and 485 oscilloscope for examples were this was
done.



On Tue, 11 Apr 2017 08:22:14 -0400, you wrote:

>I have a really naive question: how can picoamp leakage parts be relevant in 
>low impedance input pulse conditioning to an interval counter?
>
>Tim N3QE
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

David wrote:


If the 10pA specification is guaranteed by design, then wouldn't they
have to be testing the 1pA "A" parts?


That assumes the parts are produced by exactly the same process, which 
is very often not a safe assumption.  One of them may undergo extra 
process steps, for example, or one or more process steps may be 
modified.  That's not at all uncommon, BTW -- "A" versions are often the 
product of process tweaks, not selected "non-A" devices.


Best regards,

Charles


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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bob kb8tq]

Hi

If you go back in the thread, it started out as a “general purpose front end” 
design. One of the 
suggested parameters on that design was a high impedance input capability in 
the 1mega ohm range. 
Noise on a hi-z input is always an issue and input protection just makes it 
worse.  

About the only thing we have not dug into is the question of just how robust 
this or that protection 
approach is. A setup that will withstand being plugged into an European 250V 
wall outlet for 24 hours 
would likely be a bit more parts intensive than something that withstands the 
occasional exposure 
to +12V …. This all may seem a bit “nutty”. It’s worth noting that the HP 5335 
is not at all happy 
if you drive it with a 5V square wave and set the input attenuator to zero db 
(= you probably blow out
the input). 

Input protection does matter and getting it right is not a trivial thing. There 
will always be compromise. 

Bob

> On Apr 11, 2017, at 8:22 AM, Tim Shoppa  wrote:
> 
> I have a really naive question: how can picoamp leakage parts be relevant in 
> low impedance input pulse conditioning to an interval counter?
> 
> Tim N3QE
> 
>> On Apr 11, 2017, at 7:46 AM, Bob kb8tq  wrote:
>> 
>> Hi
>> 
>> 
>>> On Apr 11, 2017, at 7:05 AM, Charles Steinmetz  
>>> wrote:
>>> 
>>> David wrote:
>>> 
 I ended up qualifying 2N3904s based on manufacturer and lot and I
 think we ended up using ones from Motorola.  I wish detailed process
 information like National had was available from every manufacturer.
>>> 
>>> It is, if you ask the process engineers for it.  (From the Big Boys, that 
>>> is.  These days it seems discrete devices are being fabbed by dozens of 
>>> "garage" operations.  I can't speak for them, and wouldn't think of buying 
>>> product from them.)
>> 
>> If you dig into where your simple discrete part was made, you might be 
>> surprised. That’s even true of outfits you would
>> consider to be a “Big Guy” from days gone by. The real answer to selection 
>> today is to buy the automotive part.
>> That’s about the only thing anymore that locks down the sourcing, testing, 
>> and the process. If you buy a “normal” part,
>> it might have been made anywhere by just about any process. Yes, that’s 
>> scary and it raises a lot of questions. It is
>> a change that has happened over the last decade or two without a lot of 
>> publicity. 
>> 
>> Bob
>> 
>> 
>>> 
>>> Best regards,
>>> 
>>> Charles
>>> 
>>> 
>>> 
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>> 
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

David wrote:


I ended up qualifying 2N3904s based on manufacturer and lot and I
think we ended up using ones from Motorola.  I wish detailed process
information like National had was available from every manufacturer.


It is, if you ask the process engineers for it.  (From the Big Boys, 
that is.  These days it seems discrete devices are being fabbed by 
dozens of "garage" operations.  I can't speak for them, and wouldn't 
think of buying product from them.)


Best regards,

Charles



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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

David wrote:


I ended up qualifying 2N3904s based on manufacturer and lot and I
think we ended up using ones from Motorola.  I wish detailed process
information like National had was available from every manufacturer.


It is, if you ask the process engineers for it.  (From the Big Boys, 
that is.  These days it seems discrete devices are being fabbed by 
dozens of "garage" operations.  I can't speak for them, and wouldn't 
think of buying product from them.)


Best regards,

Charles



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Re: [time-nuts] TAPR TICC boxed (input protection)

[Tim Shoppa]

I have a really naive question: how can picoamp leakage parts be relevant in 
low impedance input pulse conditioning to an interval counter?

Tim N3QE

> On Apr 11, 2017, at 7:46 AM, Bob kb8tq  wrote:
> 
> Hi
> 
> 
>> On Apr 11, 2017, at 7:05 AM, Charles Steinmetz  wrote:
>> 
>> David wrote:
>> 
>>> I ended up qualifying 2N3904s based on manufacturer and lot and I
>>> think we ended up using ones from Motorola.  I wish detailed process
>>> information like National had was available from every manufacturer.
>> 
>> It is, if you ask the process engineers for it.  (From the Big Boys, that 
>> is.  These days it seems discrete devices are being fabbed by dozens of 
>> "garage" operations.  I can't speak for them, and wouldn't think of buying 
>> product from them.)
> 
> If you dig into where your simple discrete part was made, you might be 
> surprised. That’s even true of outfits you would
> consider to be a “Big Guy” from days gone by. The real answer to selection 
> today is to buy the automotive part.
> That’s about the only thing anymore that locks down the sourcing, testing, 
> and the process. If you buy a “normal” part,
> it might have been made anywhere by just about any process. Yes, that’s scary 
> and it raises a lot of questions. It is
> a change that has happened over the last decade or two without a lot of 
> publicity. 
> 
> Bob
> 
> 
>> 
>> Best regards,
>> 
>> Charles
>> 
>> 
>> 
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> 
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bob kb8tq]

Hi

Testing can mean a lot of different things. Did they test every single part 
they shipped for every parameter?
Did they just do a sample of parts and decide the lot was good? Did they test a 
sample of parts for a sub-set
of the specs and decide they were good? Did they test them after packaging or 
at the wafer level? Did they test
a completely different (but much easier to test) part at the wafer level and 
decide the whole wafer was good?  Did
they test one wafer out of the batch and decide the rest of the day’s 
production was good?

The further down that list you go, the cheaper the part gets. I rarely go 
looking for the most expensive part when
I’m doing a sort on the distributor site. I do toss out a few outfits I don’t 
trust, but that’s about it. I doubt I’m the only
one who shops this way. That drives the whole process to ever lower cost 
approaches. 

If you *really* need a specific parameter, test it yourself. Depending on a 
supplier to 100% test this or that is *not*
a good idea. Unless you have an agreement with them to do the testing and get 
the data from the tests, there is 
no certainty that your idea of “tested” and their idea are the same thing. 

Semiconductors are by no means unique in this regard. Your wrist watch, wall 
clock, or Cesium standard has 
the same dynamics driving it’s production. They all are impacted. That’s not 
always a bad thing. We get stuff
for less money. Other approaches to QA now drive the quality of the product 
where 100% testing once ruled. 

Bob



> On Apr 11, 2017, at 7:44 AM, Charles Steinmetz  wrote:
> 
> David wrote:
> 
>> If the 10pA specification is guaranteed by design, then wouldn't they
>> have to be testing the 1pA "A" parts?
> 
> That assumes the parts are produced by exactly the same process, which is 
> very often not a safe assumption.  One of them may undergo extra process 
> steps, for example, or one or more process steps may be modified.  That's not 
> at all uncommon, BTW -- "A" versions are often the product of process tweaks, 
> not selected "non-A" devices.
> 
> Best regards,
> 
> Charles
> 
> 
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bob kb8tq]

Hi


> On Apr 11, 2017, at 7:05 AM, Charles Steinmetz  wrote:
> 
> David wrote:
> 
>> I ended up qualifying 2N3904s based on manufacturer and lot and I
>> think we ended up using ones from Motorola.  I wish detailed process
>> information like National had was available from every manufacturer.
> 
> It is, if you ask the process engineers for it.  (From the Big Boys, that is. 
>  These days it seems discrete devices are being fabbed by dozens of "garage" 
> operations.  I can't speak for them, and wouldn't think of buying product 
> from them.)

If you dig into where your simple discrete part was made, you might be 
surprised. That’s even true of outfits you would
consider to be a “Big Guy” from days gone by. The real answer to selection 
today is to buy the automotive part.
That’s about the only thing anymore that locks down the sourcing, testing, and 
the process. If you buy a “normal” part,
it might have been made anywhere by just about any process. Yes, that’s scary 
and it raises a lot of questions. It is
a change that has happened over the last decade or two without a lot of 
publicity. 

Bob


> 
> Best regards,
> 
> Charles
> 
> 
> 
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bill Hawkins]

There are other ways that light can cause unexpected behavior.

In 1983 I worked on a process control system whose maiden installation
was in a corn processing plant, with lots of big valves and motors being
controlled. The cards that did A/D and D/A conversion of control signals
had UV erasable EPROMs for their microprocessors. There were a lot of
those cards.

One day the plant operators began complaining about the equipment
misbehaving on a large scale. The problem went away when the guy taking
flash pictures of our equipment stopped taking pictures.

We put black tape over the UV lenses.

Ob timenuts: This system later had a pulse frequency input card that I
connected to the power line. Used the operator's trending display for
process variables to watch line frequency change over time. It also had
pulse outputs, and a little work got it to play "Daisy, Daisy" like HAL
9000 in "2001: A Space Odyssey."

Bill Hawkins


-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Bob
kb8tq
Sent: Sunday, April 09, 2017 1:34 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] TAPR TICC boxed (input protection)

Hi

If anybody gets into this sort of thing in the future - There are black
/ optical blocking die coat materials out there. They are silicone based
and quite stable. 
We used a *lot* of the stuff on watch modules after it was discovered
that the watch died when exposed to a heavy dose of sunlight (right
through the LCD and into the chip . poof!!)

Bob


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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bob kb8tq]

Hi

If anybody gets into this sort of thing in the future — There are black / 
optical
blocking die coat materials out there. They are silicone based and quite 
stable. 
We used a *lot* of the stuff on watch modules after it was discovered that the
watch died when exposed to a heavy dose of sunlight (right through the LCD
and into the chip … poof!!)

Bob

> On Apr 9, 2017, at 1:10 PM, David  wrote:
> 
> I have run across the conductive carbon filled plastic problem before.
> 
> We did not actually use just paint.  We took black mastic electrically
> insulating tape, dissolved it in thinner, and painted the parts with
> it.  It dried to form a pliable black coating.
> 
> On Sat, 8 Apr 2017 17:49:01 + (UTC), you wrote:
> 
>> You need to be careful how you paint the package black.  My first 
>> electronics job was in a place that made, among other things, mass 
>> spectrometers.  We made very high input impedance electrometers for the mass 
>> specs using TO-5 can mosfet transistors.  One batch was found to be very 
>> photo sensitive through the glass/ceramic lead interface.  Someone had the 
>> idea to spray paint the bottom of the package with black paint.  Not a good 
>> idea. The black paint, likely loaded with carbon, decreased the electrometer 
>> input impedance by many orders of magnitude.  Considering that our 
>> electrometers had an input impedance of 1E-12 to 10E-15, even a fingerprint 
>> made a huge difference.  The carbon filled black paint was practically a 
>> short.
>> Maybe an overcoat with silicone or some other type of low leakage sealant, 
>> then the black paint?
>> 
>> Tom
>> 
>>> From: David 
>>> To: Discussion of precise time and frequency measurement 
>>>  
>>> Sent: Saturday, April 8, 2017 10:00 AM
>>> Subject: Re: [time-nuts] TAPR TICC boxed (input protection)
>>> 
>>> On Fri, 7 Apr 2017 01:06:17 -0400, you wrote:
>>> 
>>> controlling the offset voltage.
>>> 
>>> We ended up painting the diodes black after soldering.
>>> 
>>> I have also heard of it happening with metal TO-18 packages through
>>> the lead interface under the package.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

On Sat, 8 Apr 2017 16:30:38 -0400, you wrote:

>David wrote:
>
>> I mentioned this in connection with some manufacturers using gold
>> doping in transistors which would not normally be expected to have
>> gold doping.  So you end up with a bunch of lessor named 2N3904s which
>> meet the 2N3904 specifications but are useless if you were looking for
>> low leakage diodes.
>
>I believe all 2N3904s and 2N3906s are gold doped.  National's certainly 
>were (Processes 23 and 66), and TI's and Fairchild's are. Not heavily 
>doped, like 2N2369s (with storage times of ~20nS), but just enough to 
>bring the storage time down to ~100nS.   2N2219s, 2Ns, and 2N4401s 
>are also lightly gold doped.

I ended up qualifying 2N3904s based on manufacturer and lot and I
think we ended up using ones from Motorola.  I wish detailed process
information like National had was available from every manufacturer.

What was funny was when we did this, the lower frequency transistors
that I tested like the 2N5088/9 were much worse.

>> If [4117 leakage is] not being tested, then where is the maximum specified
>> leakage number coming from?  For a small signal bipolar transistor it
>> will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
>> shows 10pA maximum and 1pA maximum for the A versions.
>>*   *   *
>> When this discussion of low leakage input protection started, I did a
>> quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
>> and came up with nothing; all of the inexpensive JFETs are much worse
>
>Same as any "guaranteed by design" spec -- by the device design. The 
>4117 series is unlike any other JFET -- the geometry is TINY, and the 
>4117 Idss is only 30-90uA (hundreds of times lower than other low-Idss 
>JFETs). [BTW, lowest Idss is why I recommend the 4117 over the 4118 and 
>4119 for use as a low-leakage diode.  The 4118 and 4119 have higher Idss 
>-- up to 240uA for the 4118 and 600uA for the 4119 -- and tend to have 
>higher gate leakage, as well.]
>
>Best regards,
>
>Charles

If the 10pA specification is guaranteed by design, then wouldn't they
have to be testing the 1pA "A" parts?
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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

On Sat, 8 Apr 2017 21:43:31 +0200, you wrote:

>Am 08.04.2017 um 17:52 schrieb David:
>>
>> If they are not being tested, then where is the maximum specified
>> leakage number coming from?  For a small signal bipolar transistor it
>> will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
>> shows 10pA maximum and 1pA maximum for the A versions.
>
>The large print giveth and the small print taketh away.
>
>Usually there are footnotes and weasel words like "sample tested",
>"by characterisation" or "not production tested".
>The time such a small device sits on the wafer tester costs much more
>than the silicon. For 100 msec.
>At 1 pA it takes an eternity until the capacitances in the setup
>are charged. Just the waiting time makes such a diode or FET
>a premium part.

Low leakage is the defining characteristic of these JFETs so they
better be testing them.

The Calogic datasheet was the only one I checked which said anything
like "For design reference only, not 100% tested" and it did not apply
to the leakage current.

The non-A parts are only tested down to 10pA.

>Back to input protection:
>
>Someone in the sci.electronics.design group mentioned these
>< https://www.digikey.de/products/de?keywords=cmpd6001s >
>but, as usual, typical values, and watch the plot with the temperature
>as parameter. At least they are cheap.

I think these were pointed out to me before.  Since I would have to
test them to guaranty leakage below 500pA, I might as well test a
cheap small signal transistor.

If you want a laugh, take a look at NXP's various "low leakage
diodes"; they only specify and test them down to nanoamps.  But I
assume for most new EEs that *is* low leakage.

>Also interesting, while not exactly low leakage diodes, are these
>USB3 lightning arrestors:
>< https://www.digikey.de/products/de?keywords=296-25509-1-nd >
>Looks like they don't spoil the timing.
>
>regards, Gerhard

USB is not leakage sensitive.  It looks like these were only tested to
the same 100nA standard as many transistors which makes sense; they
just need to weed out bad parts.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

I have run across the conductive carbon filled plastic problem before.

We did not actually use just paint.  We took black mastic electrically
insulating tape, dissolved it in thinner, and painted the parts with
it.  It dried to form a pliable black coating.

On Sat, 8 Apr 2017 17:49:01 + (UTC), you wrote:

>You need to be careful how you paint the package black.  My first electronics 
>job was in a place that made, among other things, mass spectrometers.  We made 
>very high input impedance electrometers for the mass specs using TO-5 can 
>mosfet transistors.  One batch was found to be very photo sensitive through 
>the glass/ceramic lead interface.  Someone had the idea to spray paint the 
>bottom of the package with black paint.  Not a good idea. The black paint, 
>likely loaded with carbon, decreased the electrometer input impedance by many 
>orders of magnitude.  Considering that our electrometers had an input 
>impedance of 1E-12 to 10E-15, even a fingerprint made a huge difference.  The 
>carbon filled black paint was practically a short.
>Maybe an overcoat with silicone or some other type of low leakage sealant, 
>then the black paint?
>
>Tom
>
>> From: David 
>> To: Discussion of precise time and frequency measurement 
>>  
>> Sent: Saturday, April 8, 2017 10:00 AM
>> Subject: Re: [time-nuts] TAPR TICC boxed (input protection)
>>   
>>On Fri, 7 Apr 2017 01:06:17 -0400, you wrote:
>>
>>controlling the offset voltage.
>>
>>We ended up painting the diodes black after soldering.
>>
>>I have also heard of it happening with metal TO-18 packages through
>>the lead interface under the package.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

David wrote:


I mentioned this in connection with some manufacturers using gold
doping in transistors which would not normally be expected to have
gold doping.  So you end up with a bunch of lessor named 2N3904s which
meet the 2N3904 specifications but are useless if you were looking for
low leakage diodes.


I believe all 2N3904s and 2N3906s are gold doped.  National's certainly 
were (Processes 23 and 66), and TI's and Fairchild's are. Not heavily 
doped, like 2N2369s (with storage times of ~20nS), but just enough to 
bring the storage time down to ~100nS.   2N2219s, 2Ns, and 2N4401s 
are also lightly gold doped.



If [4117 leakage is] not being tested, then where is the maximum specified
leakage number coming from?  For a small signal bipolar transistor it
will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
shows 10pA maximum and 1pA maximum for the A versions.
   *   *   *
When this discussion of low leakage input protection started, I did a
quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
and came up with nothing; all of the inexpensive JFETs are much worse


Same as any "guaranteed by design" spec -- by the device design. The 
4117 series is unlike any other JFET -- the geometry is TINY, and the 
4117 Idss is only 30-90uA (hundreds of times lower than other low-Idss 
JFETs). [BTW, lowest Idss is why I recommend the 4117 over the 4118 and 
4119 for use as a low-leakage diode.  The 4118 and 4119 have higher Idss 
-- up to 240uA for the 4118 and 600uA for the 4119 -- and tend to have 
higher gate leakage, as well.]


Best regards,

Charles


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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

David wrote:


So gold doping does work with PNP devices.  Previously when I brought
it up, I was told gold doping only applied to NPN devices leading to
my confusion.


Since I posted, I dug through my books and found a few more references 
on point:


A couple of textbooks say commercial PNP fast switches are (were) Pt 
doped.


Motorola and National Semiconductor say they use(d) gold doping for fast 
PNP switches.  Section 6 of National's 1978 Discrete Databook ("Process 
Characteristics") lists 10 gold-doped processes (6 NPN and 4 PNP) and no 
Pt-doped processes.


Raytheon's Transistor Dice Catalog indicates that the company used gold 
doping for NPN switches (Processes CJ and CK) and Pt doping for PNP 
switches (Processes GJ, GK, and GR).


Raytheon sourced small-signal, high-speed PNP switches 2N2409, 2N2894, 
2N3640, 2N4208, 2N5771, and 2N5910 from its platinum-doped Process GR. 
National sourced these parts from its gold-doped Processes 64 and 65.


Best regards,

Charles


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Re: [time-nuts] TAPR TICC boxed (input protection)

[Gerhard Hoffmann]

Am 08.04.2017 um 17:52 schrieb David:


If they are not being tested, then where is the maximum specified
leakage number coming from?  For a small signal bipolar transistor it
will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
shows 10pA maximum and 1pA maximum for the A versions.

The large print giveth and the small print taketh away.

Usually there are footnotes and weasel words like "sample tested",
"by characterisation" or "not production tested".
The time such a small device sits on the wafer tester costs much more
than the silicon. For 100 msec.
At 1 pA it takes an eternity until the capacitances in the setup
are charged. Just the waiting time makes such a diode or FET
a premium part.


When this discussion of low leakage input protection started, I did a
quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
and came up with nothing; all of the inexpensive JFETs are much worse
until you get to premium devices.

(1) I only picked the InterFET datasheet because it was the most
readily available of the ones you mentioned.  The current Fairchild
and Linear Systems datasheets show the same thing.

Ouch, Interfet and data sheet in one sentence!  But then they could
condense it further and just give the abs.max. ratings. I have
checked out my first 7 pairs of IF3602. Some have > 100 mA
at Vgs=-0.5V, others don't have any drain current at all. I wanted
to parallel 4 pairs for noise reasons, found just 2 pairs that are
reasonably similar. At €50 a pop finding another matching 2 will cost
a pretty penny probably.

The noise spec also seems "optimistic" and there was troubling gate
current with the 2 pairs, even at Vdd=2V. The 1/f corner seems to be
OK at 30 Hz.

Back to input protection:

Someone in the sci.electronics.design group mentioned these
< https://www.digikey.de/products/de?keywords=cmpd6001s >
but, as usual, typical values, and watch the plot with the temperature
as parameter. At least they are cheap.

Also interesting, while not exactly low leakage diodes, are these
USB3 lightning arrestors:
< https://www.digikey.de/products/de?keywords=296-25509-1-nd >
Looks like they don't spoil the timing.

regards, Gerhard






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Re: [time-nuts] TAPR TICC boxed (input protection)

[Adrian Godwin]

The Siliconix PAD1 at 1pA and 0.8pF is still available :

http://www.micross.com/pdf/LSM_PAD1_TO-72.pdf

On Sat, Apr 8, 2017 at 4:52 PM, David  wrote:

> On Thu, 6 Apr 2017 22:23:43 -0400, you wrote:
>
> >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.
>
> I mentioned this in connection with some manufacturers using gold
> doping in transistors which would not normally be expected to have
> gold doping.  So you end up with a bunch of lessor named 2N3904s which
> meet the 2N3904 specifications but are useless if you were looking for
> low leakage diodes.
>
> >> 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.
>
> I thought there was a more sophisticated way but that sure sounds like
> something Tektronix would have done for grading parts.
>
> The JEDEC standard is probably what I need to find or at least start
> with.  Thank you for the tip.
>
> >> 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
>
> If they are not being tested, then where is the maximum specified
> leakage number coming from?  For a small signal bipolar transistor it
> will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
> shows 10pA maximum and 1pA maximum for the A versions.
>
> When this discussion of low leakage input protection started, I did a
> quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
> and came up with nothing; all of the inexpensive JFETs are much worse
> until you get to premium devices.
>
> (1) I only picked the InterFET datasheet because it was the most
> readily available of the ones you mentioned.  The current Fairchild
> and Linear Systems datasheets show the same thing.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Tom Curlee]

You need to be careful how you paint the package black.  My first electronics 
job was in a place that made, among other things, mass spectrometers.  We made 
very high input impedance electrometers for the mass specs using TO-5 can 
mosfet transistors.  One batch was found to be very photo sensitive through the 
glass/ceramic lead interface.  Someone had the idea to spray paint the bottom 
of the package with black paint.  Not a good idea. The black paint, likely 
loaded with carbon, decreased the electrometer input impedance by many orders 
of magnitude.  Considering that our electrometers had an input impedance of 
1E-12 to 10E-15, even a fingerprint made a huge difference.  The carbon filled 
black paint was practically a short.
Maybe an overcoat with silicone or some other type of low leakage sealant, then 
the black paint?
Tom


  From: David 
 To: Discussion of precise time and frequency measurement  
 Sent: Saturday, April 8, 2017 10:00 AM
 Subject: Re: [time-nuts] TAPR TICC boxed (input protection)
   
On Fri, 7 Apr 2017 01:06:17 -0400, you wrote:

controlling the offset voltage.

We ended up painting the diodes black after soldering.

I have also heard of it happening with metal TO-18 packages through
the lead interface under the package.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

On Thu, 6 Apr 2017 22:23:43 -0400, you wrote:

>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.

I mentioned this in connection with some manufacturers using gold
doping in transistors which would not normally be expected to have
gold doping.  So you end up with a bunch of lessor named 2N3904s which
meet the 2N3904 specifications but are useless if you were looking for
low leakage diodes.

>> 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.

I thought there was a more sophisticated way but that sure sounds like
something Tektronix would have done for grading parts.

The JEDEC standard is probably what I need to find or at least start
with.  Thank you for the tip.

>> 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

If they are not being tested, then where is the maximum specified
leakage number coming from?  For a small signal bipolar transistor it
will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
shows 10pA maximum and 1pA maximum for the A versions.

When this discussion of low leakage input protection started, I did a
quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
and came up with nothing; all of the inexpensive JFETs are much worse
until you get to premium devices.

(1) I only picked the InterFET datasheet because it was the most
readily available of the ones you mentioned.  The current Fairchild
and Linear Systems datasheets show the same thing.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

On Fri, 7 Apr 2017 01:06:17 -0400, you wrote:

>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

I got caught by this once.  We had a design which had to use hermetic
parts and this happened with the diodes used for input protection
during development and testing.  Luckily I noticed within a few
minutes that the apparent drift coincided with the angle that I was
observing the circuit leading to the discovery that my desk lamp was
controlling the offset voltage.

We ended up painting the diodes black after soldering.

I have also heard of it happening with metal TO-18 packages through
the lead interface under the package.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

On Fri, 7 Apr 2017 04:09:38 -0400, you wrote:

>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

So gold doping does work with PNP devices.  Previously when I brought
it up, I was told gold doping only applied to NPN devices leading to
my confusion.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

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


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Re: [time-nuts] TAPR TICC boxed (input protection)

[Andy]

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
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

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


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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

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.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

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


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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

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.
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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bruce Griffiths]

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  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:
> 
> https://goo.gl/LCY2vR
> 
> 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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Re: [time-nuts] TAPR TICC boxed (input protection)

[David]

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:

https://goo.gl/LCY2vR

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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Re: [time-nuts] TAPR TICC boxed (input protection)

[Charles Steinmetz]

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
.
 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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Re: [time-nuts] TAPR TICC boxed (input protection)

[Bob kb8tq]

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  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
>>> .
>>> 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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