Its somewhat difficult to achieve a 1Megohm input with just a bipolar comparator input stage especially if AC coupling is required.
Bruce > > On 20 June 2017 at 19:20 Attila Kinali <att...@kinali.ch> wrote: > > 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 > > 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.