[time-nuts] TAPR TICC boxed

Bob kb8tq kb8tq at n1k.org
Sat Apr 1 07:59:21 EDT 2017


There are low(fish) leakage / low capacitance / high speed transient suppressor diodes out there. 
The aren’t going to do anything good in a 1 megohm environment. They are quite
useful in lower impedance circuits. 


> On Apr 1, 2017, at 1:49 AM, 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
>> .
>> 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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