[sdiy] S&H

[Paul Maddox]

Tim,

> As I was telling Mr. Electronium last night, S&Hs are
> one of the more unforgiving you'll ever chase after. I
> can give some clues, but also search the various
> manufacturer's datasheets and app notes for info.

yep, the crucial thing is the aquisition time..

> First big gotcha is the on resistance of the switch,
> which forms a time constant with the SH cap and sets a
> maximum speed. The leakage around the cap sets the
> hold droop. Leakage takes many forms including opamp
> input current, switch off leakage, PCB conduction
> water, flux, etc), and of course the cap itself leaks
> internally.

This is why Id prefer a solid state soloution, its also smaller..
id rather use 32 DACs than build 32 S&Hs

> The choise of cap value is important, as
> well as type of cap. The turn-on and turn-off speed of
> the switch also goes to speed of the SH.

Im not worried about droop, as I plan to update each channel every 10uS but
I have only about 200nS of time in which to load the S&H.
for example the max5168 datasheet say it has an min aquistion time of 250nS,
so I *MIGHT* get away with it..

Time to do some more thinking..

Thanks for the help
Paul