[sdiy] How droopy is too droopy in a S & H?

[Roy J. Tellason]

On Monday 26 January 2009 02:18:11 pm John Blacet wrote:
> ---A trade off between aquisition time and hold time. .01 seems small for
> synth work. The Improbability Drive uses a 0.1 uF mylar.

Wow,  I wouldn't have thought mylar was appropriate for this application,  
though I'm not quite remembering why.

> Polystyrenes this size are huge and hard to find. Seems to work pretty well
> though. 

My recollection is that ARP and perhaps others used polycarbonate.

> S&H made up with  2 stages of a MAX 365 and a TL074 op amp. Lot's of guard
> band stuff in the PCB design!

Yup!  And no soldermask or silkscreening in at least the area of the S/H...

> > I have been seduced by the Analog Devices SMP04 quad sample and hold
> > chip. It has 4 independent S&H circuits with built in capacitors, in a
> > 16 pin chip. I claims a "low droop rate" of 2 mV per Second. Doesn't
> > that seem like a lot? How much does a normal sample & hold droop? How
> > much does an LF398 with polystyrene caps droop? Is this a waste of
> > time and money? The AD684 seems to be about 10 time better, but also
> > $50! It's hard to tell because they don't use the same units for
> > measuring the specs on all the chips...
> >
> >
> > Mark


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