Also for interest the 53131a schematic is available at http://bee.mif.pg.gda.pl/ciasteczkowypotwor/HP/53131.pdf
HP used a low input bias current bjt opamp, the Lt1008 to bias/dc servo a custom JFET buffer driving an AD96687 comparator. On Fri, Mar 31, 2017 at 10:34 PM Scott Stobbe <scott.j.sto...@gmail.com> wrote: > Fwiw, for a precision comparator you'll probably want a bipolar front end > for a lower flicker corner and better offset stability over cmos. For > high-speeds the diffpair is going to be biased fairly rich for bandwidth. > So you will more than likey have input bias currents of 100's of nA to uA > on your comparator. Which is not great with a 1 megohm source. > > On Fri, Mar 31, 2017 at 9:08 PM Charles Steinmetz <csteinm...@yandex.com> > wrote: > > Mark wrote: > > > I thought about using the clamp diodes as protection but was a bit > worried about power supply noise leaking through the diodes and adding some > jitter to the input signals... > > It is a definite worry even with a low-noise, 50 ohm input, and a > potential disaster with a 1Mohm input. Common signal diodes (1N4148, > 1N914, 1N916, 1N4448, etc.) are specified for 5-10nA of reverse current. > Even a low-leakage signal diode (e.g., 1N3595) typically has several > hundred pA of leakage. Note that the concern isn't just power supply > noise -- the leakage current itself is quite noisy. > > For low-picoamp diodes at a decent price, I use either (1) the B-C diode > of a small-signal BJT, or (2) the gate diode of a small-geometry JFET. > A 2N5550 makes a good high-voltage, low-leakage diode with leakage > current of ~30pA. Small signal HF transistors like the MPSH10 and > 2N5179 (and their SMD and PN variants) are good for ~5pA, while the gate > diode of a PN4417A JFET (or SMD variant) has reverse leakage current of > ~1pA (achieving this in practice requires a very clean board and good > layout). > > I posted some actual leakage test results to Didier's site, which can be > downloaded at > < > http://www.ko4bb.com/getsimple/index.php?id=download&file=03_App_Notes_-_Proceedings/Reverse_leakage_of_diode-connected_BJTs_and_FETs_measurement_results.pdf > >. > This document shows the connections I used to obtain the data. > > > The TICC doesn't have the resolution for it to matter or justify a > HP5370 or better quality front end. I'll probably go with a fast > comparator to implement the variable threshold input. > > Properly applied, a fast comparator will have lower jitter than the rest > of the errors, and is an excellent choice. Bruce suggested the LTC6752, > which is a great part if you need high toggle speeds (100s of MHz) or > ultra-fast edges. But you don't need high toggle rates and may not need > ultra-fast edges. Repeatability and stability are more important than > raw speed in this application. The LT1719, LT1720, or TLV3501 may work > just as well for your purpose, and they are significantly less fussy to > apply. > > Note that the LTC6752 series is an improved replacement for the ADCMP60x > series, which itself is an improved replacement for the MAX999. Of > these three, the LTC6752 is the clear winner in my tests. If you do > choose it (or similar), make sure you look at the transitions with > something that will honestly show you any chatter at frequencies up to > at least several GHz. It only takes a little transition chatter to > knock the potential timing resolution of the ultra-fast comparator way > down. Do make sure to test it with the slowest input edges you need it > to handle. > > Best regards, > > Charles > > > _______________________________________________ > 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.