[sdiy] sample and (infinite) hold

[Scott Juskiw]

>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.