sample & hold caps/ timing.

[Grant Richter]

The calculations are based on the RC time
constants formed by the internal resistance of the switch
(Ron) and the capacitor. An RC time constant describes
the amount of time it take for the DC value to go to 63%
of the final value. The rule of thumb I was taught is
the value will have settled to 99.9% after 10 RC or 10
times the time constant.

Your scanning rate has to update all output channels
in less than 1 millisecond. So take 1 millisecond
and divide by the number of output channels. That gives
you your "dwell" time at each S/H. Now find the Ron
of the multiplexer switch and chose a capacitor such
that the dwell time is 10 times the time constant.

Using a JFET input op-amp should pretty much
eliminate droop associated with the hold time.

You will probably end up with a 1,000 or 10,000 pF
hold cap. But do the math yourself, and report back
to the class your results <humor>

----------
> From: Roel Das <Roel.Das at student.groept.be>
> To: Synth DIY <synth-diy at node12b53.a2000.nl>
> Subject: sample & hold caps/ timing.
> Date: Thursday, April 13, 2000 8:25 AM
> 
> Hello again,
> 
> I was wondering.
> How do I calculate the value of the sample and hold caps for my midi CV?
> And the time the MUX should be enabled to load them?
> Which caps are best for this? From what I've read (Harry's primer on
caps)
> polystyrene film caps would be best. Or polycarbonate film. If I can find
> these in time...
> And are there nice replacements (pincompatible and better) for the tl082
> buffers?
> 
> I need calculations. They make me look good for the teachers...
> Teachers are mean.
> 
> Thanks
> Roel