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

Bob kb8tq kb8tq at n1k.org
Sun Apr 2 18:58:27 EDT 2017


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. 


> On Apr 2, 2017, at 5:12 PM, Charles Steinmetz <csteinmetz at 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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