basic S&H specs.

[Buck Buchanan]

Hi JH and all,

Thanks so much one and all for your comments on this - both on and off
list.  I'm now armed with the knowledge to make a S&H that isn't off in
left field performance wise.

More commoments below:

jhaible wrote:
>  buck wrote:
> > I'm looking for some quick opinions on sample and hold performance.
> > I've started to evaluate some circuits and realized I'm not sure what's
> > "good" for synth work.  Although I don't need extreme performance, I
> > want to be at least as good as the "average" S&H on the average synth.
> > In general I want best possible droop rate without exceeding acceptable
> > aquisition time,
> 
> May I ask you why exactly you want this ? I mean *if* you want minimum
> aquisition time and maximum hold time, then go for a 2-stage solution
> (fast analogue S&H with small capacitor, plus "digital S&H" with ADC,
> latch, and DAC).
> But "good for synth work" is something different. Most typical S&H
> applications are clocked with a few Hz (fast LFO rate), so why worry
> about low droop rate. Fast acquisition rate is more important, IMO.
> Typical values I'm using are 47nF cap plus small signal FET switch,
> or 1nF cap and CA3080 switch.

As usual I should have been more specific but my posts always seem to be
too long anyway. I'm mostly into making wierd noises (sampler fodder)
and abstract music so my needs may be away from "average".... and
wanting to hold a voltage for 15 minutes wouldn't be that absurd for
me.  Designing in a vaccume for my needs only, I could easilly come up
with a S&H that requied (for example) a 20mS acquisition time that fit
my needs perfectly.... only to find that when trying to interface with a
1mS trigger from some other synth that it will never acquire (unless I
knew in advance that a mono-stable option was needed and then it may be
too slow for the application anyway).

In this particular case, I just wanted to get the best performance from
the simple design.  Being from the weird side of things, I
wholeheartedly accept instrument limitations as character and won't cry
over S&H droop.   Untill I actually need to precisely hold indefinately,
I won't mess with the more complicated solutions - though they do sound
like fun.

Thanks again for the input expecially on what is "typical" S&H apps -
just what the doctor ordered!

Buck