[sdiy] Generating a large number of CV outputs

[Jonatan Liljedahl]

One way to reduce the noise coming from the MUX control signals is to use
grey code instead of binary count, so that only one adress line changes at
a time.

On Fri, 8 Dec 2023 at 15:29, Mike Bryant <mbryant at futurehorizons.com> wrote:

>
>    - OTOH I would love to see someone doing 100-outs MUX with single DAC spitting
>    16-bit precision at 1Msps   Roman
>
>
> I think the uneven capacitance alone to each of the S/Hs would make this
> impossible.   And the noise from all the control signals to the gates are
> almost bound to cross critical tracks.
>
> ------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Roman
> Sowa <modular at go2.pl>
> *Sent:* 08 December 2023 14:07
> *To:* Danjel van Tijn / intellijel <danjel at intellijel.com>; SDIY <
> synth-diy at synth-diy.org>
> *Subject:* Re: [sdiy] Generating a large number of CV outputs
>
> I'm triggered, because that's something I was recently evaluating (but
> don't feel like doing for various reasons).
> The DACs can be as cheap as they wish, but will never be S&H-MUX cheap.
> Surely there are cheap excellent 12-bit DACs now with price per channel
> below $1 but that's still a lot of money. You could also consider
> 96-channel DAC and just don't bother designing huge circuitry around it,
> but not only it cost $80 in singles but also is huge BGA monster.
>
> I reckon you're not doing it for space or military, so IMHO the mux way
> is the only way still.
>
> With 100 outs at "few" kHz update rate you're struggling with 1us analog
> MUX settling time, and when you say 16-bit, that makes me shiver.
> What you can do is split it to couple of separate muxes with their own
> DACs, and feed them sequentially. Huge analog circuitry with lots of
> muxes, opamps, caps and countless meters of traces will be happier
> getting an update every 10us rather than 1us. Interlace DAC update
> moments with MUX address changes to make sure new output is well settled
> before MUX opens.
> Also, with multiple DACs and MUXes you can spread the circuitry on the
> board so DAC's opamp will not have to drive too much, but only a couple
> of MUXes gathered around it.
>
> OTOH I would love to see someone doing 100-outs MUX with single DAC
> spitting 16-bit precision at 1Msps
>
> Roman
>
>
> W dniu 2023-12-08 o 01:35, Danjel van Tijn / intellijel pisze:
> > What is the current ideal way to generate a large number of CV outputs
> > from a single CPU?
> >
> > Let's say I wanted to generate 100 outputs with an update rate of a few
> > kHz and an equivalent bit depth of 12 or 16 bits.
> >
> > I know that back in the day when DACs were extremely expensive, the
> > designs would use a single DAC and then demux to a bunch of S&Hs.
> >
> > Now that DACs are cheaper, is it better to chain a bunch of 8- or
> > 16-channel DACs?
> >
> > Or would it make more sense to use an FPGA and create a giant
> > multi-channel 1-Bit DAC?
> >
> > Looking at some Elektron hardware teardowns it appears that they are
> > using the latter approach.
> >
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-- 
/Jonatan
http://kymatica.com
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