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

[Roman]

what about charge injection?
at MAX306's specified typical 2pC and your chosen 2n2 S&H cap
it will cause voltage error of about 1mV. At 10pC max it goes wild
causing huge 5mV error! ;)

Is the charge injection constant? Does it change direction? Can it
be trimmed out? Or DAC could write 5mV less accounting for
expected MAX's influence?

Roman

----- Original Message -----
From: Seb Francis <seb at is-uk.com>
To: patchell <patchell at silcom.com>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, October 12, 2002 4:03 AM
Subject: Re: [sdiy] Getting back to my Midi->CV converter...


> 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
> > > ------------------------------------------------
> > >
> > >
> > >
>