Hi I think the purpose is to get high input impedance for high frequency signal. The input impedance of comparator is high at low frequency range only.
Li Ang / BI7LNQ ---Original--- From: "Attila Kinali"<att...@kinali.ch> Date: 2017/6/20 15:20:44 To: "Discussion of precise time and frequency measurement"<time-nuts@febo.com>; Subject: [time-nuts] SR620/PM66xx/CNT-90 input stages Hi, I had a look at the PM668x[1] and CNT-90[2] schematics yesterday and noticed one thing: The input stages are strikingly similar to the SR620[3] (down to the parts used) and all of them have a gain 1 amplifier infront of the comparator. The PM668x service manual explicitly calls it "impedance converter", even. Now, if the downstream circuit would be low impedance, I could understand that, but the sink is a comparator with a high impedance input (only a few µA input current). I am sure the engineers had a good reason to add those amplifiers, but I cannot guess why. Would someone be so kind and enlighten me? Attila Kinali [1] http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/Fluke_PM6681_Service_Manual.pdf [2] http://assets.fluke.com/manuals/6690____smeng0000.pdf [3] http://www.ko4bb.com/getsimple/index.php?id=download&file=02_GPS_Timing/Stanford_Research_Systems/SR620_Universal_Time_Interval_Counter_Schematics.pdf -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson _______________________________________________ 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.