[sdiy] Getting back to my Midi->CV converter...

[Seb Francis]

As it goes, I've just been doing the S&H capacitor maths for my own MIDI 2 CV, maybe this saves you the trouble ...

My DAC accuracy is 1/2 LSB at 14bit (about 0.5mV with FSR of 15V - it needs to be -7.5V->7.5V for the digital envelopes)
Desired accuracy is within 1mV (just over a cent at 1V/Octave)

So desired S&H accuracy is 0.5mV

Taking 2.2nF caps as a starting point
MAX306 MUX max resistance is 175R

t - time in seconds
V - "good enough" final voltage (14.9995V)
Vi - DAC voltage (15V)

V=Vi * ( 1 - e^(-t/RC) )
t = -ln(1 - V/Vi) * RC
~= 4us for the above cap and a FSR of 15V

So I figured 5us is enough time to wait for the S&H to settle.  It could be reduced by using smaller caps, but I want to have digital LFOs and envelopes, and the PIC will be busy enough calculating these during the 5us pause (25 whole instructions!).  And anyway, I guess the output could end up a bit "stepped" if the caps were too small.
I'm planning on refreshing the 15 S&Hs pretty frequently in order to get reasonably smooth LFOs up to 100Hz if possible.  If you are refreshing much slower (e.g. full S&H cycle once per ms) then bigger caps might be better to avoid droop - I didn't do any calculations for this yet.

Personally I'm using polystyrene caps (and low-offset, low-drift opamps of course)

Seb



Grant Richter wrote:

> Bear in mind the refresh rate is also the latency time. You need to refresh
> every channel once a millisecond. Also use polycarbonate caps for the sample
> and holds.
>
> I build a large scanned sample and hold array (80 channels) for testing
> infared temperature detectors. Getting 12 bit accuracy at high speed was
> quite a challenge. You can really see dielectric absorption in action!
>
> > From: patchell <patchell at silcom.com>
> > Date: Thu, 10 Oct 2002 21:46:35 -0700
> > To: "synth-diy at dropmix.xs4all.nl" <synth-diy at dropmix.xs4all.nl>
> > Subject: [sdiy] Getting back to my Midi->CV converter...
> >
> > http://www.silcom.com/~patchell/miditocv/miditocv.html
> >
> > I noticed I originally posted that page about 4 years ago...my how
> > time flies.
> >
> > The web page gives a very brief description of what I plan on doing,
> > plus, I posted a schematic of the FPGA I just got to compile that is
> > going to do the DMA services for updating the sample/holds...yep, I said
> > DMA...just call me crazy Jim...it has been a while since I did a DMA
> > (about 15 years), so I decided to so this to get my feet wet again
> > before tackling the nasty one I am going to start at work.
> >
> > --
> > -Jim
> > ------------------------------------------------
> > * Visit:http://www.silcom.com/~patchell/
> > *-----------------------------------------------
> > *I'm sure glad Merry Christmas comes just once a year
> > * -Yogi Yorgensen
> > ------------------------------------------------
> >
> >
> >