[time-nuts] TAPR TICC boxed

Charles Steinmetz csteinmetz at yandex.com
Fri Mar 31 21:00:33 EDT 2017


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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