[time-nuts] TAPR TICC boxed

Scott Stobbe scott.j.stobbe at gmail.com
Fri Mar 31 23:36:18 EDT 2017

Also for interest the 53131a schematic is available at

HP used a low input bias current bjt opamp, the Lt1008 to bias/dc servo a
custom JFET buffer driving an AD96687 comparator.

On Fri, Mar 31, 2017 at 10:34 PM Scott Stobbe <scott.j.stobbe at gmail.com>

> 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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