[sdiy] DG409 blues

[David G. Dixon]

Does anybody have any experience with or insight into the DG408/9 analog
multiplexers?  Specifically, I would like to know whether it is OK to put
+15V into the logical inputs.  I've wired it up that way in my custom
version of the CGS analog shift register, and I seem to be having problems.
The switch seems to be introducing random voltages into the sampled levels.
I know this is not a problem with the high-impedance lines, because the held
voltages do not vary with time, at least as far as I can detect with my
ears.  They only change when the switch is switching.

When I look at the datasheet, it always shows 2.4V or 5V going into the
logical inputs, and presents data for the device when the logical inputs are
held either 2.4V or 0V.  What I'm wondering is, is this just convention, or
does it actually matter to the performance of the device whether that input
is held at TTL-compatible levels or not.  I know the switch works at the
high level, but I'm wondering whether this is the source of my problems.  I
also have not put any resistors between the rail and the input, so perhaps
I'm overdriving the input with too much current.

In any case, if anyone has any insight into this little beastie, I'd love to
hear it.  I want to get this ASR design working properly.