FJH1100
Ultra Low Leakage Diode
Alex
On 3/31/2017 6:00 PM, Charles Steinmetz 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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