Attempting sub nanosecond timing with an actual 1Mohm source is an exercise in futility. There are very few cases where one would want to attempt precision timing measurements with such a high impedance source. The 1M pulldown on the TICC input is merely intended to maintain a valid logic input should the user leave that input disconnected. In actual use with PPS signals the source impedance is in most cases a few tens of ohms. If one wishes to have a 1Mohm input impedance for use with AC coupled signals then a low noise FET input buffer preceding the comparator is required.
Protection diodes in this application not only need to have low leakage, they also need to turn on and off fast enough to be useful. The propagation delay dispersion (both vs common mode and vs overdrive) also need to be considered along with the comparator jitter. Bruce > > and overdrive (both vs overdrive and vs input common modeOn 01 April > 2017 at 15:34 Scott Stobbe <scott.j.sto...@gmail.com> wrote: > > Fwiw, for a precision comparator you'll probably want a bipolar front end > for a lower flicker corner and better offset stability over cmos. For > high-speeds the diffpair is going to be biased fairly rich for bandwidth. > So you will more than likey have input bias currents of 100's of nA to uA > on your comparator. Which is not great with a 1 megohm source. > > On Fri, Mar 31, 2017 at 9:08 PM Charles Steinmetz <csteinm...@yandex.com> > 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 > > > > 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 > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
