basic S&H specs.

[Harry Bissell]

Hi Buck (and all...)

Buck Buchanan wrote:

> Hi everybody,
>
> I checked the archvies, Allen Strange book, and others on this but not
> finding it (but I know it must be out there).
>
> 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, problems with interafacing to other synth's triggers,
> or going to more complex topologies such as NS AN-294 (though AN-294
> seems like a fun thing to try).
>
> So, solicitiing opinions on any or all of the following:
>
> -maximum acceptable aquisition time and related trigger width.

Do you have a two switch keyboard ??? If so you can have the voltage stable
before the gate signal comes... and the gate removed (hold) before the
voltage is removed. This works really well.

If you can get to the final value in 1-2mS your ear will not hear anything
but a step response... (IMHO)

>
> -width of trigger on "most" synths?

probably 1-5mS

>
> -what's "good" or "average" droop rate?

One you can't hear ??? My synth stays on pitch for 10-15 minutes at a time.
If you replay the same note at the end of 10 min... well... its the same
note. Good enough for me.

>
> -performance of std 4016 w/3140 vs LF398 (leaning towards LF398 due to
> board space limitations)

Don't know the LF398. I use the 4016 and CA3140. My cap is a 0.15uF
polystyrene
Becuase I gate (Track/Hold) the acquisition rate isn't too critical. I
can't play very fast... the shortest note I could hit acquires just fine.

>
> -source on polystyrenes >.01uf (this is where Mouser and Newark
> selection stops). They are getting harder to find. I'd try Southern
> Electronics Corporation (was F-dyne). The cap I'm using was a "Plessy
> Minibox" but they are long gone AFAIK. Maybe you can get a sample... I
> don't know of a distributor that handles small quantities. BTW the F-dyne
> caps are BIG (in board space) maybe 1.2" long by .5" dia for a .1uF....
> (they are 1% very good quality...)

Polystyrene has dielectric absorption of .02 (as does polypropylene and
teflon)
Polycarbonate has .08
Polyester (Mylar) has .20 (avoid)

Tempco are not usually a problem.

>
> -National gives same performance curves for polystyrenes and
> polypropylenes... is this true?

sounds about right....

>
> -anything else ya wanna throw out there.
>
> Thanks mucho one and all and happy new year!
>
> Buck