[time-nuts] TAPR TICC boxed

Alex Pummer alex at pcscons.com
Fri Mar 31 21:39:11 EDT 2017

Ultra Low Leakage Diode


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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