[sdiy] sample and (infinite) hold

[Harry Bissell Jr]

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.
>