[time-nuts] TAPR TICC boxed

Bob Camp kb8tq at n1k.org
Sat Apr 1 12:48:03 EDT 2017


Hi

The whole delay difference thing does get into a “do you care?” sort of category. The 
testing process you are doing may well calibrate out (or ignore) an offset of this nature. 
This is quite true in a number of TimeNut sort of tests. 

Bob

> On Apr 1, 2017, at 4:02 AM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> 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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>>>> 
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