[sdiy] sample and (infinite) hold

[rfahl at pdx.edu]

I believe the Polyfusion (1048?) used this approach... both stages were 
triggered at the same time, the first cap being a higher value, the second 
much lower. 

Anyone out there who's used one of these modules that can testify how well it 
holds its load?

R.

Quoting Harry Bissell Jr <harrybissell at prodigy.net>:

> One technique is to use two S/H in series.
> the first is set for a fast acquisition time
> (small S/H cap) and the second a long time
> (large cap).  One shot multivibrators control
> the relative sample apertures.   I think National
> Semi might have had this as an app note but I don't
> remember for sure.
> 
> H^) harry
> 
> 
> --- Scott Juskiw <scott at tellun.com> wrote:
> 
> > >You can do 20 minutes with analogue. You can't do
> > 20 days, or the 
> > >full length of a Klaus Schulze piece. :-)
> > >
> > >One thing to consider is circuit board leakage.
> > Some designs put a 
> > >guard track around the hold cap which helps keep
> > the charge in.
> > 
> > I recall reading about those S&H guard rings
> > somewhere. I haven't 
> > gotten to the PCB layout yet, I'm still testing and
> > looking for bugs.
> > 
> > >If the droop seems more or less constant, the
> > easiest way to 
> > >compensate for it is to add a charge top-up circuit
> > that injects a 
> > >very small current into the cap to make up for any
> > losses. This is 
> > >an approach that's prone to all sorts of errors -
> > it's impossible to 
> > >build a circuit that will be accurate across a full
> > range of 
> > >temperatures - but it's better than a passive hold.
> > 
> > I think that's a bit beyond me, I think the ADC/DAC
> > route is easier 
> > on my tiny brain.
> > 
> > >The digital alternative with an ADC/DAC combination
> > is the only 
> > >solution that can provide infinite hold. But done
> > properly it's 
> > >certainly not cheap. You could probably get away
> > with an 8-bit 
> > >ADC/DAC combination for key voltages.
> > >
> > >However - if you have a quantizer you already have
> > this circuit. Can 
> > >you not lock the quantizer output so it only
> > changes within a short 
> > >window triggered by a key on?
> > 
> > I was just using the quantizer as an example, it's
> > actually not part 
> > of the circuit. The S&H is the output from a ribbon
> > controller. I 
> > like to use the ribbon to dial up a voltage that
> > (eventually) gets 
> > fed to a VCO or VCF frequency input. I'm just
> > finding it rather 
> > annoying that the voltage slowly creeps down and
> > things eventually go 
> > out of tune. I'm thinking I'll try the ADC/DAC
> > route. Thanks for your 
> > help.
> > 
> 
>