[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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