Hi If you go back in the thread, it started out as a “general purpose front end” design. One of the suggested parameters on that design was a high impedance input capability in the 1mega ohm range. Noise on a hi-z input is always an issue and input protection just makes it worse.
About the only thing we have not dug into is the question of just how robust this or that protection approach is. A setup that will withstand being plugged into an European 250V wall outlet for 24 hours would likely be a bit more parts intensive than something that withstands the occasional exposure to +12V …. This all may seem a bit “nutty”. It’s worth noting that the HP 5335 is not at all happy if you drive it with a 5V square wave and set the input attenuator to zero db (= you probably blow out the input). Input protection does matter and getting it right is not a trivial thing. There will always be compromise. Bob > On Apr 11, 2017, at 8:22 AM, Tim Shoppa <[email protected]> wrote: > > I have a really naive question: how can picoamp leakage parts be relevant in > low impedance input pulse conditioning to an interval counter? > > Tim N3QE > >> On Apr 11, 2017, at 7:46 AM, Bob kb8tq <[email protected]> wrote: >> >> Hi >> >> >>> On Apr 11, 2017, at 7:05 AM, Charles Steinmetz <[email protected]> >>> wrote: >>> >>> David wrote: >>> >>>> I ended up qualifying 2N3904s based on manufacturer and lot and I >>>> think we ended up using ones from Motorola. I wish detailed process >>>> information like National had was available from every manufacturer. >>> >>> It is, if you ask the process engineers for it. (From the Big Boys, that >>> is. These days it seems discrete devices are being fabbed by dozens of >>> "garage" operations. I can't speak for them, and wouldn't think of buying >>> product from them.) >> >> If you dig into where your simple discrete part was made, you might be >> surprised. That’s even true of outfits you would >> consider to be a “Big Guy” from days gone by. The real answer to selection >> today is to buy the automotive part. >> That’s about the only thing anymore that locks down the sourcing, testing, >> and the process. If you buy a “normal” part, >> it might have been made anywhere by just about any process. Yes, that’s >> scary and it raises a lot of questions. It is >> a change that has happened over the last decade or two without a lot of >> publicity. >> >> Bob >> >> >>> >>> Best regards, >>> >>> Charles >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
