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

Tom Bugs admin at bugbrand.co.uk
Sat Dec 15 11:58:27 CET 2018

For single switches try DG417/8/9
DG417 is SPST NC
DG418 is SPST NO
DG419 is SPDT

Useful little chips.

On 14/12/2018 20:09, David G Dixon wrote:
> I never have problems with analog electronics.  In the case of this 
> scanner, for example, I designed the analog part so that there are 
> plateaus in the control voltage when switching occurs, which gives me 
> about 200mV of slop in the CV for switching. This circuit worked the 
> very first time (even though I've rebuilt it three times).  The 
> problem here was all digital.
> Later today, I'm ordering some DG444 and DG445, and also some single 
> SPST switches (don't know the DG number yet) and I'm going to rebuild 
> this thing Roman's way (with a few minor modifiations).  The analog 
> part is perfect and won't have to change at all.
> On the digital side, I'm worried about the fact that the LM3914 
> comparators do not have built-in hysteresis, and there is no way to 
> install it, so I'm worried that the comparators will oscillate.  I'm 
> going to stick with LM339, fully rigged for hysteresis (about 10mV 
> worth) as this worked well.  Does anyone here have any experience with 
> oscillating LM3914?
>     ------------------------------------------------------------------------
>     *From:* Rutger Vlek [mailto:rutgervlek at gmail.com]
>     *Sent:* Friday, December 14, 2018 5:36 AM
>     *To:* David G Dixon; SDIY List
>     *Subject:* Re: [sdiy] Help, I'm Desperate! (Charge Injection with
>     DG408)
>     | It just shouldn't be this hard.
>     Ha, I totally agree! I once explored an approach to
>     ring-modulation using a single VCA and an inverting switch. It
>     seemed like a nice, cost-effective, approach (especially since I
>     wanted to use a discrete VCA that would be too bulky for another
>     approach). After two prototypes of this circuit, I simply gave up.
>     In the analog domain it was simply too difficult to trigger the
>     switch exactly at the point where the VCA was fully closed, due to
>     circuit accuracy and noise. It was a hard lesson in the reality of
>     analog electronics. I went down the route of simply duplicating
>     the discrete circuit for the negative quadrants, and it works
>     extremely well albeit not super-linear, but that was a deliberate
>     choice.
>     Regards,
>     Rutger
>     Op wo 12 dec. 2018 om 03:56 schreef David G Dixon
>     <dixon at mail.ubc.ca <mailto:dixon at mail.ubc.ca>>:
>         It just shouldn't be this hard.
>         ------------------------------------------------------------------------
>         *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org
>         <mailto:synth-diy-bounces at synth-diy.org>] *On Behalf Of *Roman
>         *Sent:* Tuesday, December 11, 2018 1:20 PM
>         *To:* synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>
>         *Subject:* Re: [sdiy] Help, I'm Desperate! (Charge Injection
>         with DG408)
>             I know it's too late, as the circuit at this stage has
>             already gone slightly different path, and I already gave
>             up, but couldn't help drawing this little schemo that
>             explains my idea of using single switches with that
>             obsoleted chip. Switches should be DG444 or similar
>             (0=closed, 1=open), or replace diodes with NAND gates and
>             use any regular dirt cheap switch like 4066.
>             There's no switching happening while VCA is open, provided
>             that VCA control triangles match the edges of LM3914
>             stages. The only switched channels are the ones routed to
>             muted VCA, so in theory there should be no clicking audible.
>             Basicaly this is just the circuit that makes this input
>             switching sequence:
>             VCA-A:12233445566..
>             VCA-B:11223344556...
>             by creating one step overlap between stages. Only 6 inputs
>             in this example, but can be extended to any other number.
>             http://www.synthdiy.eu/files/scanner.png
>             IMHO switching active channel will always produce some
>             clicking for many reasons: not matched channels,
>             difference between transistion times H->L and L->H as Ingo
>             said, break-before-make feature of DG407, slow response of
>             4532, and who knows if not also charge injection.
>             Roman
>             Dnia 11 grudnia 2018 18:24 Ingo Debus <igg.debus at gmail.com
>             <mailto:igg.debus at gmail.com>> napisał(a):
>                     Am 11.12.2018 um 08:10 schrieb David G Dixon
>                     <dixon at mail.ubc.ca <mailto:dixon at mail.ubc.ca>>:
>                     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).
>                 Couldn’t this still be caused by a very brief
>                 „forbidden state“ during the transition from 011 to
>                 100? In your case, the forbidden state would be 000
>                 (MSBit changes slower that the other two) or 111
>                 (MSBit changes faster). Probably just a difference
>                 between rising and falling slope. Can you check with a
>                 scope?
>                 Ingo
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