[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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