Hi The whole delay difference thing does get into a “do you care?” sort of category. The testing process you are doing may well calibrate out (or ignore) an offset of this nature. This is quite true in a number of TimeNut sort of tests.
Bob > On Apr 1, 2017, at 4:02 AM, Bruce Griffiths <bruce.griffi...@xtra.co.nz> > wrote: > > The common mode propagation delay dispersion is also likely to be significant > unless one uses an SiGe ECL/CML comparator. > > Calibrating this or actually the differential dispersion between channels is > an interesting but not insoluble issue. > > Bruce > >> >> On 01 April 2017 at 18:49 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 >>> >>> 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. _______________________________________________ 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.