sample & hold caps/ timing.

[Dan Gendreau]

> How do I calculate the value of the sample and hold caps for my midi CV?

This is off the top of my head, so somebody holler if I make a mistake...

The most important factor here is your output buffer opamp. Check the
datasheet for the opamp you will be using. There should be a term for the
input resistance. It will be a really large value. For the TL084, the term
is called ri on page 8:
http://www-s.ti.com/sc/psheets/slos081e/slos081e.pdf

Now:
Tau = R[+input] * C[buffer cap]

In this case, ~5*Tau is the time it takes for your buffer cap to discharge
completely.

Check your electronics book for the equation for RC decay. Its some
exponential decay thing (dont have a book here to look it up). This will
give you an equation for the voltage across your cap vs. time. Using the
equation, you can calculate how far your cap will decay (at max DAC output
Voltage) from the close of the gate, to the the next time you open it for
this output. This will tell you the maximum amount of ripple at your S/H
buffer output.

Now the key is to select a value of C which gives output ripple which is
"significantly" smaller than 1LSB of your DAC output. I would probably
suggest 1/100th LSB, and this may be overkill, but I did say at "Least".

> And the time the MUX should be enabled to load them?

Same thing here:
Tau = RC.

In this case though, the R is the resistance between the DAC voltage source
and the top of your buffer cap, C is the same buffer cap as above and ~5*Tau
is the time it takes the DAC voltage source to charge your buffer cap to the
desired voltage. Dont forget to include the resistance of the gate between
them(it usually has a resistance of like 20 Ohm, but again, check the data
sheet).

The time you should leave the gate open is at _Least_ 5*Tau. You can
probably neglect the input resistance of your output buffer opamp in this
case.

Now Part one above gives you the lower constraint of C. ie. It must be at
least a certain value to prevent ripple. Part two above gives you the upper
constraint of C. ie. It cant be larger than a certain value, else you wint
have enough time to charge it.

> I need calculations. They make me look good for the teachers...
> Teachers are mean.

Depends on how you define mean. My electronics teacher was like an army
drill sergeant. He didnt take any slacking from anyone and we all loved him
for it. :)

Hope that gets you going in the right direction,
-Dan Gendreau