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

ulfur hansson ulfurh at gmail.com
Tue Dec 11 14:19:57 CET 2018


what an intense saga!!

cannot wait to play around with the fruits of your labor dave, hats off to you for persevering!

best,
-ú

Sent from outer space

> On Dec 11, 2018, at 2:10 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> OK, so to continue the saga:
> 
> I rebuilt my circuit with a flash-ADC comparator chain, and it all worked
> reasonably well, except that when the CV was stuck at 0V (right in the
> middle, where my fader has a detent) one of the comparators was oscillating
> like a mo-fo, and this was causing the DG408s to freak right out, putting
> all sorts of noise onto the audio output.
> 
> So, I decided to put hysteresis on the comparators.  This required an input
> resistor (10k) to the + input of each comparator, plus a large hysteresis
> resistor (2.2M).  I also added 10k resistors to the - inputs, so added 21
> resistors to the circuit overall.  Somehow, I managed to lay this all out
> with just 0.4" of additional length.  I didn't want to rebuild the entire
> board to test this, so I built a daughterboard with just the two LM339s and
> the CD4532 on it, and I removed all of these components from the main board,
> and wired up the daughterboard at seven connection points.  This worked
> great, and the daughterboard floated above the main board on the stiffness
> of the connecting wires.
> 
> Result?  Now it works ALMOST perfectly.  The noise has disappeared (as I
> knew it would) and the comparators do not oscillate at all.  There is now
> only one tiny problem: There is still an almost imperceptible click when the
> CV crosses 0V (and the logic control voltage crosses 2.5V) in the positive
> direction.  This is when the logic switches from 011 to 100 (i.e., all three
> bits change).  Interestingly, I don't hear the click at all when the CV goes
> in the other direction (100 to 011).  Hence, what I think I am hearing now
> is just charge injection from the A2 bit, which only occurs in one direction
> according to a document I found on the web about it.  Also, the click is
> very faint, and could thus be about 20pC of charge as per the datasheet.
> 
> So, I believe that I have now eliminated all of the problems I had
> introduced to this circuit with my naive approach to logic design.  I don't
> know if I'll be able to get rid of this tiny click, but I'm going to try two
> things:
> 
> 1) Adding 10nF caps to ground from the switch outputs, as suggested in the
> datasheet and the document I found, and
> 2) Reducing the input impedance on the downstream circuitry from 100k to
> 10k, which the datasheet suggests should help (lower impedance is better).
> 
> According to what I've read, these things should reduce the effect of charge
> injection.  (If it is charge injection, then I would have been no better off
> with digital SPST switches.)  However, this is going to have to wait until
> tomorrow afternoon.
> 
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