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

David G Dixon dixon at mail.ubc.ca
Thu Dec 6 19:01:41 CET 2018

> Why bother encoding 8 inputs into 3 address and then decoding 
> that back in a mux since 3914 gives you all what's needed for 
> an input scanner.

1) The 3914 is set up to drive LEDs, and I didn't want to have to figure out
how to use it to generate logic levels.  I found the datasheet to be
somewhat cryptic in that regard.  It's probably easy to do (with one
resistor per comparator), but I just couldn't be bothered learning how.

2) The 3914 has 10 comparators, and I only want 7 (or 8, but the lowest one
is always on), so I was going to have to figure out how to alter the voltage
range into the chip so that 5V fell exactly between the 8th and 9th
comparators.  Again, it's forcing a chip to do something that it wasn't
really designed to do.  The only part of that chip I wanted was the
comparators, and so why not just use comparators?  Also, with the 7
comparators, the voltage range I need is exactly given by my 5V reference
and 8 1% 10k resistors, so I know it will be accurate.

3) The 3914 is already obsolete.

4) The 3914 generates the equivalent of reverse logic (because the
comparators suck current when on), which would require interfacing with the
'148, which also generates reverse logic, but the DG408 requires positive
logic.  One is just the other in the wrong direction, but I didn't want to
deal with that.  Hence, I preferred to use the 4532, which accepts and
generates positive logic.  The 4532 is nicely driven by comparators.

5) The "mux" here is a pair of 8-channel analog switches which are
processing audio signals.  I'm not decoding the 3-bit code.  I'm using it to
drive those switches.

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