[time-nuts] TAPR TICC boxed (input protection)

Gerhard Hoffmann dk4xp at arcor.de
Sat Apr 8 15:43:31 EDT 2017

Am 08.04.2017 um 17:52 schrieb David:
> If they are not being tested, then where is the maximum specified
> leakage number coming from?  For a small signal bipolar transistor it
> will typically be 25nA, 50nA, or 100nA, but the InterFET datasheet (1)
> shows 10pA maximum and 1pA maximum for the A versions.
The large print giveth and the small print taketh away.

Usually there are footnotes and weasel words like "sample tested",
"by characterisation" or "not production tested".
The time such a small device sits on the wafer tester costs much more
than the silicon. For 100 msec.
At 1 pA it takes an eternity until the capacitances in the setup
are charged. Just the waiting time makes such a diode or FET
a premium part.

> When this discussion of low leakage input protection started, I did a
> quick search for inexpensive alternatives to the 4117/4118/4119 JFETs
> and came up with nothing; all of the inexpensive JFETs are much worse
> until you get to premium devices.
> (1) I only picked the InterFET datasheet because it was the most
> readily available of the ones you mentioned.  The current Fairchild
> and Linear Systems datasheets show the same thing.
Ouch, Interfet and data sheet in one sentence!  But then they could
condense it further and just give the abs.max. ratings. I have
checked out my first 7 pairs of IF3602. Some have > 100 mA
at Vgs=-0.5V, others don't have any drain current at all. I wanted
to parallel 4 pairs for noise reasons, found just 2 pairs that are
reasonably similar. At €50 a pop finding another matching 2 will cost
a pretty penny probably.

The noise spec also seems "optimistic" and there was troubling gate
current with the 2 pairs, even at Vdd=2V. The 1/f corner seems to be
OK at 30 Hz.

Back to input protection:

Someone in the sci.electronics.design group mentioned these
< https://www.digikey.de/products/de?keywords=cmpd6001s >
but, as usual, typical values, and watch the plot with the temperature
as parameter. At least they are cheap.

Also interesting, while not exactly low leakage diodes, are these
USB3 lightning arrestors:
< https://www.digikey.de/products/de?keywords=296-25509-1-nd >
Looks like they don't spoil the timing.

regards, Gerhard

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