[time-nuts] SR620/PM66xx/CNT-90 input stages
bruce.griffiths at xtra.co.nz
Tue Jun 20 05:57:48 EDT 2017
Its somewhat difficult to achieve a 1Megohm input with just a bipolar comparator input stage especially if AC coupling is required.
> On 20 June 2017 at 19:20 Attila Kinali <attila at kinali.ch> wrote:
> I had a look at the PM668x and CNT-90 schematics yesterday and
> noticed one thing: The input stages are strikingly similar to the
> SR620 (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
>  http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/Fluke_PM6681_Service_Manual.pdf
>  http://assets.fluke.com/manuals/6690____smeng0000.pdf
>  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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