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

David G Dixon dixon at mail.ubc.ca
Fri Dec 7 21:33:17 CET 2018

Hello Roman,

Thanks for your thoughtful replies.  I do appreciate them.

I actually have some 3914s, which I bought for some purpose which I now
can't remember.

As far as the reference voltage, it would have to be larger than 5V (about
6.25V, I think), which means that I would have to amplify my 5V reference
with another opamp, because I wouldn't take it straight from the rail.  I
already have a buffered 5V reference which I am using all over the board, so
it was just easier to use that.

At the end of the day, the two 339s take up about the same space as the 3914
(for which I have no sockets on hand), and the only real inconvenience is
that they require 8 resistors for the external voltage ladder, which would
be internal to the 3914, but they don't require any other opamps or
resistors.  Hence, the 3914 really represented no added convenience here.
It might have saved me 4 or 5 resistors, but cost me another TL072, and I
would have had to jerry-rig a socket.

The DG444 is an inappropriate chip for my circuit.  I need two 8-channel
multiplexers.  The DG407 would have been ideal, but I only had DG408s, so I
used two of those.  As far as the reverse logic, this is no big deal, since
the binary sequences are exactly backwards.  All I would have had to do was
to reverse the polarity of my CV, or just re-number the inputs in reverse

In any case, the layout is done, and I'm just about to go up to the printer
and make the board.


> -----Original Message-----
> From: Roman Sowa [mailto:modular at go2.pl] 
> Sent: Friday, December 07, 2018 5:53 AM
> To: David G Dixon; synth-diy at synth-diy.org
> Subject: Re: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)
> 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

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