[time-nuts] TAPR TICC boxed

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 1 04:02:47 EDT 2017


The common mode propagation delay dispersion is also likely to be significant unless one uses an SiGe ECL/CML comparator.

Calibrating this or actually the differential dispersion between channels is an interesting but not insoluble issue.

Bruce 

> 
>     On 01 April 2017 at 18:49 Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
>     Also interesting the LTC6752 is rail-rail input. Any rail-rail input opamp
>     I've used ends up with an ugly bump in input offset voltage transitioning
>     from the nmos or npn diff pair to the pmos or nmos. I'm not sure how good
>     or bad a rail-rail comparator may behave when common-mode biased in that
>     region.
> 
>     On Fri, Mar 31, 2017 at 11:22 PM Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>     wrote:
> 
>         > > 
> >         Attempting sub nanosecond timing with an actual 1Mohm source is an
> >         exercise in futility. There are very few cases where one would want to
> >         attempt precision timing measurements with such a high impedance source.
> >         The 1M pulldown on the TICC input is merely intended to maintain a valid
> >         logic input should the user leave that input disconnected. In actual use
> >         with PPS signals the source impedance is in most cases a few tens of ohms.
> >         If one wishes to have a 1Mohm input impedance for use with AC coupled
> >         signals then a low noise FET input buffer preceding the comparator is
> >         required.
> > 
> >         Protection diodes in this application not only need to have low leakage,
> >         they also need to turn on and off fast enough to be useful.
> > 
> >         The propagation delay dispersion (both vs common mode and vs overdrive)
> >         also need to be considered along with the comparator jitter.
> > 
> >         Bruce
> > 
> >         and overdrive (both vs overdrive and vs input common modeOn 01 April 2017
> >         at 15:34 Scott Stobbe <scott.j.stobbe at 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 <csteinmetz at 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 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
> > 
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