Last word on caps

[harrybissell at prodigy.net]

Cap size is determined by application... but in general the longer you need to hold, the larger the cap muct be,
and the faster you need to acquire a sample, the smaller the cap has to be.

If you are sampling a very fast moving signal, a large cap will "filter" out all the data you wanted...

In all cases, higher impedance amplifier inputs and low offset voltages are "good" things.

You can sample a fast signal with a S/H made with a small cap, and then (while holding that one for a "little" while) sample that S/H with a much larger cap to get rid of droop.

In my analog keyboard S/H (where the data is a slow as I can wiggle my fingers) I use a .15uF polystyrene cap driving a CA3140 op amp... Droop is essentially unmeasurable (come back in 1/2 hour its the same pitch.

OTOH: most "flying" multiplexers using the 4051 analog switch seem to like caps about .01uF. Remember that the switch resistance forms a low pass filter.

:^) Harry


 ---- On Dec  7 "Hugo Haesaert" <hugo.haesaert at skynet.be> wrote: 
> Hi All !
> 
> But what determines the value/size of the cap used ?
> 
> NPO gets good marks from Bob, but is not readily available in 0.1 µF, 
> the value mostly seen for multiplexed dac outputs .  And styrene caps 
> only go to 56nF at my supplier .
> 
> The longer the hold time required, the larger the cap, the kbd/SH cap 
> of the Roland System 100 is huge .  Otoh, my MKS80 (it's fine Paul :) 
> uses tiny ceramic discs for s/h caps (followed by TL064's, now why's 
> that ?) .
> 
> Hmm...
> 
> Later .
> 
> 
> Keep 'em oscillating :)
> 
> Hugo
> =