[time-nuts] TAPR TICC boxed
Alex Pummer
alex at pcscons.com
Fri Mar 31 21:39:11 EDT 2017
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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