S&H

[Tony Clark]

>     One classic S/H uses two op amps, FET ones preferably, both are wired as
> voltage followers, the output of the second feeds the inverting input of the
> first.  The second just feeds back to its own inverting input.  Between the
> first and second is a switch and the cap to ground.
> 
>     National puts that all in an 8-pin package for you.  Its called the LF398.
> Digikey lists it for $3.33 in onesies.  Looks like the simplest, and maybe the
> cheapest solution.

   If you do this, the best part to use is the CA3240.  As we all know 
how the 3140 is used for VCO's, the input impedance is extremely well 
suited for S&H applications.
   I just used such a circuit in a new product I'm developing for my 
company.  Works great, but I will warn you that even a circuit with this 
high of an impedance and proper circuit layout will still drift, but it 
is relatively small (0.1mV per minute or so).  I used a .47uF metallized 
polypropylene capacitor which is fairly inexpensive and has a small 
dissipation factor.
   If cost wasn't a factor, I'd use a larger cap since my application 
doesn't require a super fast integration time.  There's always a tadeoff 
of integration time vs. hold time.

   Good luck!

   Tony

------------------------------------,----------------------------------
I can't drive (my Moog) 55!         |     The E-Music DIY Archive
------------------------------------|  
Tony Clark -- clark at andrews.edu     | aupe.phys.andrews.edu/diy_archive
http://aupe.phys.andrews.edu/~clark |     Contributions welcomed!
------------------------------------'----------------------------------