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

[Roman Sowa]

Before it turns into pointless flame war, let me say "I quit". I'm not 
here to tell anybody how to make things. Just wanted to express my 
admiration for simplicity of 3914 with SPST.

But since you took the time to nicely comment in such detail, it would 
be rude to leave this unanswered. So inline then...

> 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.

That's open collector outputs. No magic in there.

> 
> 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

Scale the input voltage accordingly. You scale it somehow already, so 
just change resistor values.

> 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.

Luckily, 3914 also contains voltage reference!
And I would never expect to hear "the chip wasn't designed for it" from 
someone using VCA as VCO core ;)

> 
> 3) The 3914 is already obsolete.

Is it going to be produced in thousands over span of several years? I 
think you mentioned it's one-off custom job. Still plenty of 3914 around.

> 
> 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.

That's why I mentioned DG444

> 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.

I meant that DG408 is actually a decoder too.

Roman