[time-nuts] TAPR TICC boxed

[Scott Stobbe]

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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