[sdiy] Generating a large number of CV outputs

[Mattias Rickardsson]

On Sun, 10 Dec 2023 at 20:36, brianw <brianw at audiobanshee.com> wrote:

> A passive RC link between the mux cap (a.k.a. S/H cap) before the buffer
> op-amp will drain the S/H storage cap.


Yes, but it will only drain charge to the next cap, not to ground or other
potential.


> I'm super-paranoid about selecting op-amps with JFET inputs and tens of
> gigaohms of input impedance to avoid draining that S/H cap. Putting a
> passive RC filter between the cap and op-amp buffer would defeat that.
>

I'd say it works well. :-)


> Wouldn't the same RC filter work equally well *after* the buffer op-amp?
>

Sometimes, but not if it's supposed to feed low-impedance destinations.
IIRC I've added an RC link after the op-amp as well in some critical
situation.

/mr


On Dec 10, 2023, at 3:50 AM, Mattias Rickardsson <mr at analogue.org> wrote:
> > brianw <brianw at audiobanshee.com> skrev:
> > On Dec 8, 2023, at 6:50 AM, Mattias Rickardsson <mr at analogue.org> wrote:
> > > Roman Sowa <modular at go2.pl> skrev:
> > >> OTOH I would love to see someone doing 100-outs MUX with single DAC
> > >> spitting 16-bit precision at 1Msps
> > >
> > > That's basically what we do since 2012.
> > > 16-bit 1MSPS DAC running at 0.96 MHz, multiplexing its output to 40 CV
> channels, each at 24kHz update rate (no zipper noise in the audio band).
> > > Works well, but I wouldn't want to try controlling that DAC/mux timing
> with a microprocessor, or build it on a single-sided board. :-)
> >
> > One question I have is, "How much reconstruction filtering is necessary?"
> >
> > It was enough with an extra RC link directly after the mux cap, before
> the buffer op-amp. Added cost: ~0.008 $ per CV channel.
> >
> > /mr
> >
>
>
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