[sdiy] S&H

[harrybissell]

Hi John...

I don't think so (in most cases).  It would depend on the topology of the S/H.
Many "music" S/H are an open loop system... where the capacitor charges
at the RC constant of the switch "on" resistance and the capacitor value.
These
don't have any feedback...

Those that employ feedback usually overcome the switch on-resistance by making
the applied voltage much higher than the desired value, then switching the
charging
current off when the proper value is achieved. Its faster and can overshoot and

ring. Feedback capacitor might help here to stabilize the system.

Droop from opamp offset voltages or bias currents probably can't be
compensated,
but can be minimized by making the cap big with respect to the currents
involved.

I tried an LM324 as a buffer amp (in a non critical S/H) and boy did I get a
big
surprise.  The input PNP likes to source current to the hold cap...resulting in

POSITIVE droop.

Cool... like having a car that makes gas... just stop every couple of miles and

drain off a gallon or two...   ;^P

H^) harry

John L Marshall wrote:

> Tim,
>
> Can't the droop be compensated for by a second capacitor in the feedback
> path of the buffer opamp?
>
> Take care,
> John
> ---------------------------------------------------------------------------