[sdiy] sample and (infinite) hold

[Roy J. Tellason]

On Wednesday 05 January 2005 01:05 am, Scott Juskiw wrote:
> I've built a S&H circuit using one of those dedicated LF398 S&H
> chips. It works as expected, but I'm finding the droop to be
> unacceptable; I need a S&H with infinite hold. I searched the net and
> found a couple of references like this one:
>
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/12
>
> which explains how to accomplish this using an ADC followed by a DAC.
> But then I looked at the prices of those ADC chips, gadzooks! I can't
> even find a source for the chips mentioned in the app note (beyond
> free samples). I'm hoping there's a cheaper solution.
>
> Has anyone built a sample and infinite hold and can shed some light
> in my direction?

Infinite is a really long time...

I've worked with some of that stuff,  and it's crucial that you follow the 
right construction practices there,  and that you choose the proper type of 
capacitor for your hold.

What kind of a cap did you use?

The stuff I built,  I used a dual FET as was done in something or other I ran 
into at the time (ARP?  I forget),  but that was before FET-input op amps 
became commonplace.  I don't know if they'd work or not,  but the use of them 
seems to be pretty common.

What kind of droop are you seeing and in what kind of a time frame?