[sdiy] Generating a large number of CV outputs

[brianw]

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. 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.

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

p.s. After the op-amp is where I placed my single-pole low-pass. So I guess you've answered my question in respect to whether some folks consider single-pole to be enough.

Brian


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
>