Hi There are low(fish) leakage / low capacitance / high speed transient suppressor diodes out there. The aren’t going to do anything good in a 1 megohm environment. They are quite useful in lower impedance circuits.
Bob > On Apr 1, 2017, at 1:49 AM, Scott Stobbe <scott.j.sto...@gmail.com> wrote: > > Also interesting the LTC6752 is rail-rail input. Any rail-rail input opamp > I've used ends up with an ugly bump in input offset voltage transitioning > from the nmos or npn diff pair to the pmos or nmos. I'm not sure how good > or bad a rail-rail comparator may behave when common-mode biased in that > region. > > On Fri, Mar 31, 2017 at 11:22 PM Bruce Griffiths <bruce.griffi...@xtra.co.nz> > wrote: > >> 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 >> >> . >> 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. _______________________________________________ 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.