[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

David G Dixon dixon at mail.ubc.ca
Thu Dec 6 09:55:25 CET 2018

Today I designed a new version of my scanner circuit which uses a
7-comparator cascade from two LM339s to a CD4532 8-to-3 bit priority encoder
to form a 3-bit flash ADC.  The output of the 4532 drives the two DG408
switches.  I am confident that this will work well and be more or less
switching-noise free.  The speed of the comparators is completely irrelevant
in this case, as only one is ever changing at any given time.  The only
thing that matters is that the 3 output bits of the 4532 must be reasonably
synchronous, and there is no reason to believe otherwise, since they all
share more or less the same gate structure.  I don't fully understand the
propogation delay specs in the CD4532 datasheet, but I have to assume that
they relate to the time it takes to change the output code when a change is
exerted at the data pins, and not any delay between successive bits turning
on or off.  In any case, the proof will be in the pudding, as they say.
I've laid out a new board, but I won't get a chance to build and test it
until Friday, so y'all will just have to think about something else between
now and then.


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