[time-nuts] SR620/PM66xx/CNT-90 input stages

[Bruce Griffiths]

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 <attila at 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
> 
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