[sdiy] sample & hold

[Ian Fritz]

At 12:59 PM 2/2/06, Scott Stites wrote:
>I Shoulda been more specific - I was wondering if the outputs could track 
>CV well enough with a fairly long period before droop became noticable, 
>say, when controlling a VCO.  By polyphony, I meant having the ability to 
>rotate through the S&H elements to control four overlapping voices from a 
>monophonic control source, but that was really the red herring of a poorly 
>formed question.

Got it.  In agreement with Jim, for the polyphony application, if you are 
continually updating the S/H's, then this design would be an overkill.  As 
for droop, I was shooting for no audible VCO droop over 10 sec or so and I 
was able to achieve that.  The only problem here is that it is important 
that the OTA control pin is really pulled hard low in the hold state.  I 
use a series transistor switch for the sampling pulse and I had to add a 1M 
resistor from its collector to the negative supply to get the lowest 
possible droop.  The sampling time is around 50 us and the sampling cap is 
1800 pF, so the circuit runs well up into the audio range (>5 kHz) also.

   Ian