simple sample/hold circuit?

[patchell]

    If I am not mistaken, the Paia circuits just uses a CD4051.  This, a
polystyrene or polypropelene capacitor, and a J-Fet follower would be probably
the easiest.  I don't know what the leakage is on a CD4051, but this is one
case where a little static damage can go a long way to making a sample and hold
that drifts a lot.  Also, this is a case where board cleanliness is of the
utmost importance.  Gaurd Rings might also help in this area as well, although,
that might be going just a bit overboard.   The size of the capacitor as you
noted will affect the droop time, but it also affects the settling time.  The
rule of thumb I use is that the gate time on the sample and hold needs to be 10
times the time constant of the resultant RC circuit that will be made up of the
Dac output impeadence, the switch resistance and the holding capacitor.  That
is generally addaquate time to settle.  And make sure you use one of the above
type dialectric capacitors.  A bad dialectric will make the sample and hold
just terrible.

    -Jim

Andy Sloane wrote:

> I'm thinking of making a quick, cheap midi2cv converter with multiple outs
> using a PIC16C84 and a single DAC.. Like a PIC version of PAiA's design
> (which IMHO is a little overcomplex).  My question is, what's a good way to
> do sample and hold on the DAC output so that I can scan through all the
> outputs using a single DAC?  I guess it doesn't need to hold for very long,
> but I haven't written the PIC code yet so I don't know how long it needs to
> hold for.
>
> My electronic devices book suggests a design simply using a MOSFET and a
> cap.  Is this the best way to go?  I've never really experimented with
> mosfets before.  Obviously you'd need an opamp follower after that or
> something... So are there any other s&h tricks I should know?  Can you do
> this with BJTs somehow?  or opamps?
>
> -Andy.



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